Port z4c solver into latest available version of master.

Merge request for the z4c solver into master. The changes are as minimal as possible and the kernels of the original code are used for the fundamental parts. The major additions are the finite difference stencils used for discrete derivatives, defined in

src/utils/finite_diff.hpp

and the usage of athena tensor structures, defined from the athena array structures in

src/athena_tensor.hpp

These additions are anyway restricted to the parts of the code concerning the z4c solver.

A detailed (but probably not completely exhaustive) report of the porting of z4c solver in athenak is in porting_report.md

It seems that there are few conflicts that are due to the extension of z4c-specific lines in the following files. It should be fairly easy to merge manually. Conflicts: src/outputs/basetype_output.cpp src/outputs/outputs.hpp src/outputs/restart.cpp src/pgen/pgen.cpp