Setting precision

[fedepedron]

We should probably set a better way for real-type precision.
@Nick-Chaos already added selected_real_kind support, but also using iso_C_bindings might be
useful considering g2g.

Relevant links:
http://fortranwiki.org/fortran/show/Real+precision
https://stackoverflow.com/questions/16370206/numerical-precision-in-fortran-95
https://software.intel.com/en-us/forums/intel-fortran-compiler/topic/702716
https://stackoverflow.com/questions/3170239/fortran-integer4-vs-integer4-vs-integerkind-4/3170438#3170438