Fortran compiler flags

Change the Fortran compiler flags so that they still can be changed from
the Cmake command-line. Add special flags for Release and Debug builds,
increase the optimization level for release builds (to the Cmake default
for 'gfortran' and to the same level C++ code is compiled with).

In addition one might think about adding ```-Wall```, ```-Werror```, and ```-fimplicit-none``` (then at least implicit variables would need to be specified in each function) to the Fortran flags. The last two options will certainly break the build.