ENH: Allow writing HDF5 transforms as ITK transforms

[cookpa]

HDF5 vector images don't work with transform readers.

[cookpa]

Feels wrong to put a file type dependence into antsApplyTransforms.cxx, but it works.

The other option would be to continue writing H5 files as vector images, and modify itkantsReadWriteTransform.h so that it tries to read an HDF5 file as a displacement field first. This strategy would avoid any file type testing in antsApplyTransforms, but the resulting .h5 files still won't be ITK transforms. I guess we could add a separate program to do that.

[cookpa]

Addressing #1315

[gdevenyi]

but the resulting .h5 files still won't be ITK transforms.

I think I'd rather we write out ITK transforms, so they can be loaded/used elsewhere with minimal effort.

[ntustison]

Thanks @cookpa . I think @gdevenyi has a good point---if anybody is using ANTs with hdf5 files, I imagine they're using them elsewhere as well.

[cookpa]

Thanks - I will merge this after some more tests.

[cookpa]

Fixes #1315