Correct an error in image_expressions_cpp.py

See merge request mgenet/dolfin_dic!6

corrections for box image generation

See merge request mgenet/dolfin_dic!4

Homogeneous deformation now based on Green-Lagrange (ok only for tension/compression) OR deformation gradient (always ok)

Get part id_array and iarray_sector_id in the case where ref_frame is not None

See merge request mgenet/dolfin_dic!3

Add the case where the image is deformed and the displacement comes from a vtu file

See merge request mgenet/dolfin_dic!2

Need to instantiate before use, right?

See merge request mgenet/dolfin_dic!1

Main function has been split into mutliple classes. fedic2 has the same interface as original fedic function, but uses the new architecture.