Solve refacto

[Breush]

• Method::CRA nows really holds the iteration method and also a dispatch type (whether to use MPI) ;
• broke solve solutions into different files according to the method used ;
• renamed some CRA structures to be more consistent (CraBuilder etc.) ;
• replaced specifier's matrix shape by a flag (and removed extra symmetric info) ;
• reinterpret_cast<> are used instead of copying the methods ;
• make check, make benchmarks, make examples are all right
• make fullcheck is strictly equivalent to master's

[ClementPernet]

Please add a treatment for RationalTag. This is tested in examples/solverat.C and examples/sparsesolverat.C.

[Breush]

@ClementPernet Ok, I fixed the tests, everything is fine for me.

[ClementPernet]

LGTM, MERRRGIIINNNG!