Only import itk when we need it
Only import itk when we need it

The cost of importing itk is significant so only import in functions
that use it.

OpenChemistry/tomviz,cryos/tomviz,mathturtle/tomviz,cryos/tomviz,thewtex/tomviz,OpenChemistry/tomviz,cjh1/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,thewtex/tomviz,cjh1/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,cjh1/tomviz,thewtex/tomviz,cryos/tomviz