ENH: ExternalData: Use ITK series parse expression by default
ENH: ExternalData: Use ITK series parse expression by default

ITK baseline images are named with the form

  $test.png
  $test.1.png
  $test.2.png

where "$test" may end in a digit.  Fix the default series parse
expression to handle this form.

PlutoniumHeart/ITK,malaterre/ITK,heimdali/ITK,richardbeare/ITK,CapeDrew/DCMTK-ITK,atsnyder/ITK,msmolens/ITK,hendradarwin/ITK,CapeDrew/DCMTK-ITK,ajjl/ITK,heimdali/ITK,hinerm/ITK,PlutoniumHeart/ITK,thewtex/ITK,hinerm/ITK,jcfr/ITK,LucHermitte/ITK,stnava/ITK,Kitware/ITK,ajjl/ITK,eile/ITK,atsnyder/ITK,hjmjohnson/ITK,hendradarwin/ITK,biotrump/ITK,fuentesdt/InsightToolkit-dev,paulnovo/ITK,fbudin69500/ITK,hendradarwin/ITK,BRAINSia/ITK,rhgong/itk-with-dom,paulnovo/ITK,fedral/ITK,paulnovo/ITK,BRAINSia/ITK,Kitware/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,jmerkow/ITK,hendradarwin/ITK,itkvideo/ITK,fuentesdt/InsightToolkit-dev,thewtex/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,malaterre/ITK,cpatrick/ITK-RemoteIO,itkvideo/ITK,rhgong/itk-with-dom,heimdali/ITK,BlueBrain/ITK,fuentesdt/InsightToolkit-dev,rhgong/itk-with-dom,BlueBrain/ITK,blowekamp/ITK,blowekamp/ITK,BlueBrain/ITK,hinerm/ITK,fbudin69500/ITK,eile/ITK,LucasGandel/ITK,hendradarwin/ITK,jcfr/ITK,biotrump/ITK,vfonov/ITK,wkjeong/ITK,LucHermitte/ITK,rhgong/itk-with-dom,zachary-williamson/ITK,eile/ITK,fuentesdt/InsightToolkit-dev,spinicist/ITK,PlutoniumHeart/ITK,cpatrick/ITK-RemoteIO,wkjeong/ITK,PlutoniumHeart/ITK,fbudin69500/ITK,atsnyder/ITK,fuentesdt/InsightToolkit-dev,jcfr/ITK,zachary-williamson/ITK,fbudin69500/ITK,LucHermitte/ITK,atsnyder/ITK,hinerm/ITK,stnava/ITK,fedral/ITK,stnava/ITK,wkjeong/ITK,BRAINSia/ITK,richardbeare/ITK,ajjl/ITK,biotrump/ITK,biotrump/ITK,itkvideo/ITK,paulnovo/ITK,thewtex/ITK,paulnovo/ITK,Kitware/ITK,Kitware/ITK,jmerkow/ITK,hinerm/ITK,BlueBrain/ITK,LucasGandel/ITK,cpatrick/ITK-RemoteIO,malaterre/ITK,fedral/ITK,LucasGandel/ITK,ajjl/ITK,fuentesdt/InsightToolkit-dev,fedral/ITK,hinerm/ITK,hendradarwin/ITK,zachary-williamson/ITK,ajjl/ITK,blowekamp/ITK,PlutoniumHeart/ITK,heimdali/ITK,hjmjohnson/ITK,blowekamp/ITK,rhgong/itk-with-dom,hjmjohnson/ITK,atsnyder/ITK,jmerkow/ITK,BRAINSia/ITK,paulnovo/ITK,fedral/ITK,eile/ITK,richardbeare/ITK,biotrump/ITK,malaterre/ITK,CapeDrew/DCMTK-ITK,biotrump/ITK,blowekamp/ITK,BRAINSia/ITK,rhgong/itk-with-dom,blowekamp/ITK,fuentesdt/InsightToolkit-dev,ajjl/ITK,hjmjohnson/ITK,GEHC-Surgery/ITK,spinicist/ITK,stnava/ITK,LucasGandel/ITK,GEHC-Surgery/ITK,fedral/ITK,vfonov/ITK,msmolens/ITK,LucasGandel/ITK,CapeDrew/DCMTK-ITK,hjmjohnson/ITK,LucHermitte/ITK,itkvideo/ITK,blowekamp/ITK,itkvideo/ITK,heimdali/ITK,atsnyder/ITK,fuentesdt/InsightToolkit-dev,vfonov/ITK,hjmjohnson/ITK,jmerkow/ITK,biotrump/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,wkjeong/ITK,fbudin69500/ITK,vfonov/ITK,spinicist/ITK,GEHC-Surgery/ITK,jcfr/ITK,msmolens/ITK,heimdali/ITK,stnava/ITK,zachary-williamson/ITK,malaterre/ITK,hendradarwin/ITK,spinicist/ITK,stnava/ITK,thewtex/ITK,msmolens/ITK,msmolens/ITK,thewtex/ITK,richardbeare/ITK,wkjeong/ITK,Kitware/ITK,richardbeare/ITK,cpatrick/ITK-RemoteIO,malaterre/ITK,fbudin69500/ITK,itkvideo/ITK,GEHC-Surgery/ITK,eile/ITK,InsightSoftwareConsortium/ITK,BlueBrain/ITK,CapeDrew/DCMTK-ITK,hinerm/ITK,paulnovo/ITK,wkjeong/ITK,zachary-williamson/ITK,wkjeong/ITK,atsnyder/ITK,fbudin69500/ITK,stnava/ITK,zachary-williamson/ITK,spinicist/ITK,paulnovo/ITK,fuentesdt/InsightToolkit-dev,Kitware/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,jmerkow/ITK,cpatrick/ITK-RemoteIO,eile/ITK,richardbeare/ITK,spinicist/ITK,jmerkow/ITK,LucHermitte/ITK,zachary-williamson/ITK,msmolens/ITK,GEHC-Surgery/ITK,hinerm/ITK,rhgong/itk-with-dom,LucHermitte/ITK,biotrump/ITK,LucasGandel/ITK,malaterre/ITK,CapeDrew/DCMTK-ITK,GEHC-Surgery/ITK,atsnyder/ITK,hjmjohnson/ITK,atsnyder/ITK,InsightSoftwareConsortium/ITK,jcfr/ITK,InsightSoftwareConsortium/ITK,jcfr/ITK,itkvideo/ITK,GEHC-Surgery/ITK,BlueBrain/ITK,malaterre/ITK,CapeDrew/DCMTK-ITK,heimdali/ITK,eile/ITK,msmolens/ITK,jcfr/ITK,spinicist/ITK,CapeDrew/DCMTK-ITK,richardbeare/ITK,vfonov/ITK,wkjeong/ITK,LucHermitte/ITK,vfonov/ITK,vfonov/ITK,fedral/ITK,zachary-williamson/ITK,zachary-williamson/ITK,cpatrick/ITK-RemoteIO,ajjl/ITK,GEHC-Surgery/ITK,hinerm/ITK,CapeDrew/DCMTK-ITK,jcfr/ITK,eile/ITK,BlueBrain/ITK,cpatrick/ITK-RemoteIO,blowekamp/ITK,cpatrick/ITK-RemoteIO,fbudin69500/ITK,jmerkow/ITK,vfonov/ITK,jmerkow/ITK,LucasGandel/ITK,itkvideo/ITK,stnava/ITK,spinicist/ITK,rhgong/itk-with-dom,LucHermitte/ITK,PlutoniumHeart/ITK,ajjl/ITK,hendradarwin/ITK,spinicist/ITK,itkvideo/ITK,BlueBrain/ITK,PlutoniumHeart/ITK,eile/ITK,stnava/ITK,Kitware/ITK,thewtex/ITK,fedral/ITK,heimdali/ITK,PlutoniumHeart/ITK,thewtex/ITK,msmolens/ITK