DOC: Update documentation graph font to avoid warnings
DOC: Update documentation graph font to avoid warnings

Recent versions of Doxygen do not include the FreeSans font for use in
generating graphs, and default to Helvetica anyway when FreeSans is not
found. By specifying Helvetica in the configuration file, a warning is
fixed.

Change-Id: I2991416c3dcb958709df928ad929694711c85b25

PlutoniumHeart/ITK,richardbeare/ITK,fbudin69500/ITK,BlueBrain/ITK,biotrump/ITK,BlueBrain/ITK,LucasGandel/ITK,vfonov/ITK,heimdali/ITK,jmerkow/ITK,jcfr/ITK,heimdali/ITK,malaterre/ITK,jcfr/ITK,fedral/ITK,richardbeare/ITK,richardbeare/ITK,msmolens/ITK,thewtex/ITK,LucasGandel/ITK,msmolens/ITK,fedral/ITK,Kitware/ITK,stnava/ITK,zachary-williamson/ITK,eile/ITK,atsnyder/ITK,thewtex/ITK,ajjl/ITK,BRAINSia/ITK,malaterre/ITK,atsnyder/ITK,eile/ITK,LucHermitte/ITK,fbudin69500/ITK,Kitware/ITK,stnava/ITK,LucHermitte/ITK,thewtex/ITK,blowekamp/ITK,LucHermitte/ITK,hendradarwin/ITK,hendradarwin/ITK,thewtex/ITK,vfonov/ITK,msmolens/ITK,BRAINSia/ITK,fedral/ITK,malaterre/ITK,atsnyder/ITK,LucasGandel/ITK,zachary-williamson/ITK,spinicist/ITK,hendradarwin/ITK,heimdali/ITK,LucasGandel/ITK,zachary-williamson/ITK,richardbeare/ITK,atsnyder/ITK,blowekamp/ITK,ajjl/ITK,ajjl/ITK,zachary-williamson/ITK,spinicist/ITK,eile/ITK,BlueBrain/ITK,vfonov/ITK,atsnyder/ITK,jcfr/ITK,PlutoniumHeart/ITK,fedral/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,PlutoniumHeart/ITK,InsightSoftwareConsortium/ITK,stnava/ITK,LucasGandel/ITK,malaterre/ITK,biotrump/ITK,stnava/ITK,jmerkow/ITK,malaterre/ITK,eile/ITK,atsnyder/ITK,blowekamp/ITK,PlutoniumHeart/ITK,jmerkow/ITK,ajjl/ITK,stnava/ITK,richardbeare/ITK,blowekamp/ITK,fbudin69500/ITK,LucHermitte/ITK,LucasGandel/ITK,msmolens/ITK,hendradarwin/ITK,LucHermitte/ITK,fbudin69500/ITK,spinicist/ITK,stnava/ITK,zachary-williamson/ITK,ajjl/ITK,BlueBrain/ITK,LucHermitte/ITK,atsnyder/ITK,eile/ITK,richardbeare/ITK,BRAINSia/ITK,vfonov/ITK,hjmjohnson/ITK,hjmjohnson/ITK,vfonov/ITK,PlutoniumHeart/ITK,InsightSoftwareConsortium/ITK,zachary-williamson/ITK,zachary-williamson/ITK,jmerkow/ITK,Kitware/ITK,ajjl/ITK,atsnyder/ITK,hjmjohnson/ITK,thewtex/ITK,jcfr/ITK,vfonov/ITK,BRAINSia/ITK,BRAINSia/ITK,fedral/ITK,InsightSoftwareConsortium/ITK,PlutoniumHeart/ITK,LucasGandel/ITK,biotrump/ITK,BlueBrain/ITK,Kitware/ITK,biotrump/ITK,hendradarwin/ITK,jcfr/ITK,biotrump/ITK,fedral/ITK,blowekamp/ITK,vfonov/ITK,BRAINSia/ITK,biotrump/ITK,spinicist/ITK,blowekamp/ITK,BRAINSia/ITK,spinicist/ITK,LucHermitte/ITK,Kitware/ITK,spinicist/ITK,fedral/ITK,blowekamp/ITK,spinicist/ITK,zachary-williamson/ITK,msmolens/ITK,eile/ITK,biotrump/ITK,fbudin69500/ITK,hendradarwin/ITK,zachary-williamson/ITK,atsnyder/ITK,stnava/ITK,hendradarwin/ITK,fedral/ITK,eile/ITK,heimdali/ITK,malaterre/ITK,jmerkow/ITK,thewtex/ITK,spinicist/ITK,jcfr/ITK,fbudin69500/ITK,malaterre/ITK,msmolens/ITK,biotrump/ITK,heimdali/ITK,jmerkow/ITK,blowekamp/ITK,BlueBrain/ITK,malaterre/ITK,hendradarwin/ITK,BlueBrain/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,heimdali/ITK,jmerkow/ITK,heimdali/ITK,hjmjohnson/ITK,stnava/ITK,jmerkow/ITK,fbudin69500/ITK,spinicist/ITK,heimdali/ITK,jcfr/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,msmolens/ITK,Kitware/ITK,msmolens/ITK,thewtex/ITK,hjmjohnson/ITK,eile/ITK,Kitware/ITK,ajjl/ITK,stnava/ITK,vfonov/ITK,ajjl/ITK,PlutoniumHeart/ITK,vfonov/ITK,LucHermitte/ITK,fbudin69500/ITK,InsightSoftwareConsortium/ITK,eile/ITK,richardbeare/ITK,malaterre/ITK,PlutoniumHeart/ITK,jcfr/ITK,BlueBrain/ITK