Replace literal constants with parameters

[gold2718]

There are many literal constants in Oslo Aero that should be documented.
Some of them should be replaced with shared parameters such as the constant, 287.04, in src/aero_model.F90 (should be rair from physconst) while others should be turned into module or subroutine parameters with appropriate names and source documentation.

Note that changing 287.04 to rair will change answers so this should be done in a separate commit.

[gold2718]

Also, all constants (parameters or literal) should be explicitly type _r8. One such file that needs fixing is src/oslo_areo_ocean.F90 .

[oyvindseland]

Yes I agree and given the development phase it is the correct time to do so.

Do you want help with going through the variables?
I think we should do the cleaning up in 2.5. Or should we also do it for older versions? I presume given the rewriting from 2.1 to 2.3 we can do the work in 2.3 and then merge.

[gold2718]

I think it depends on how the scheduling falls out. Since some of this work might change answers, it should be in a separate commit (so we can verify that nothing bad has happened and we are just dealing with tiny changes). I would say it is a lower priority than getting 2.3 released so I would like to finish the other 2.3 work first.