Diagnostics support

[bartgol]

This PR addresses issue #283 . Highlights:

• Diagnostics are now computed in C++, with views on host that store a F90 pointer.
• Unless explicitly diabled (via disable_diagnostics=true), diagnostics are always computed on the last time step, regardless of statefreq. Hence, for perf runs, you need to explicitly disable diagnostics.
• The cxx_vs_f90 tests now perform 2 checks: the usual cprnc test on nc files, and a check on the raw outputs, looking for diagnostics lines (so fare they are all and only the lines containing the string '/dt'), and comparing f90 against cxx.

[bartgol]

Note: there is some code in F90 that can be purged, since it is no longer present in homme. That's for another PR though, where we could clean also implicit stuff.