ENH: Allow specification of FFT engine in Python binding of Projection classes. Added parallel tests for Projection and MaterialLinearElastic.

Summary:
MAINT: Moved 'lengths' from FFTEngine to Projection

FFTEngine never used the lengths parameter, just stored it. The Projection is the only place where the physical size of the domain plays a role.

MAINT: resolutions->subdomain_resolutions, locations->subdomain_locations

MAINT: lengths->domain_lengths

Merge branch 'feat/mpi2'

ENH: Added resolutions and locations getters.

MAINT: Moved Engine to first template argument.

MAINT: Added Engine as a separate template parameter.

Merge branch 'master' of ssh://c4science.ch/source/muSpectre

ENH: Projection has now option to specify FFT engine.

MAINT: Make mpi4py optional.

Merge branch 'master' into feat/mpi2

ENH: Added resolutions, locations and lengths getters.

MAINT: Throw exception if input array size does not match field size.

TST: Added parallel projection test.

CMAKE: Added Python MPI test.

TST: Added PFFT tests

ENH: Added resolutions, locations and lengths getters.

TST: Added MPI parallel linear elastic material check.

Merge branch 'master' of ssh://c4science.ch/source/muSpectre

Merge branch 'feat/mpi2'

MAINT: Disable MPI communicator parameter in Python interface if compiled without MPI.

Test Plan: test pass

Reviewers: junge

Reviewed By: junge

Differential Revision: https://c4science.ch/D181