add tests between different SIRFImageData types

[KrisThielemans]

• add VoxelisedGeometryInfo::operator== (use accuracy as in https://github.com/CCPPETMR/SIRF/blob/12d2a9dc36f63efc854e1ac5bb293e3ee6e15a1c/src/Registration/cReg/NiftiImageData.cpp#L837)

• write != in terms of ==

• add SIRFImageData::operator== (and !=)

  1. compare voxelised_geometry_info
  2. check norm(diff)/norm(arg1)< 1.E-3

• add ctest in cSynergistic:

  • test is:

  im1=read filename with engine1
  im2= otherengine.ImageData(im1)
  im1==im2;
  

  • to do this, copy lots of lines from https://github.com/CCPPETMR/SIRF/blob/master/src/Synergistic/sirf_convert_image_type.cpp and a bit of https://github.com/CCPPETMR/SIRF/blob/master/src/Synergistic/tests/test_cSynergistic.cpp (later, use create C++ function to read image with engine specified as text #525)

• expose to Matlab/Python and add tests there. Need to add test on norm of as_array as well.

[evgueni-ovtchinnikov]

@rijobro: So far as I can see otherengine can only be Reg at present - or is there a branch that has other conversions implemented?

[rijobro]

yes unfortunately we can only convert to NiftiImageData, not the other way round.

PR for ImageData->Gadgetron #537,
PR for ImageData->STIR #535.

[KrisThielemans]

I guess the point is to have the test in master, then merge it into the PRs above, and extend it to the other cases.