Add real8 support to CMake

[letmaik]

Implements #11.

This needs some more testing.

[dmey]

That's fine -- I will look at this once #9 is in.

[dmey]

OK with me. Conditional to full checks passing after #9 is merged in.

[dmey]

We should add at least build tests for this as well before merging this in.

[letmaik]

I enabled build tests via Travis CI and Appveyor, where half of the configurations now build with USE_REAL8=1. This prevents adding another dimension to Azure Pipelines where we already have a lot of jobs.

Given the comments in #11, we should probably in the README change

|USE_REAL8|ON, OFF|OFF|Whether to use 8-byte reals|

to

|USE_REAL8|ON, OFF|OFF|Whether to use double precision reals|

I would still keep the name USE_REAL8 simply to be in line with what WRF does (-r8). But I'm open to suggestions. It could also be USE_DP for example.

Or we leave it as it is since we don't support platforms/compilers where real8 is not actually 8-byte reals...

[dmey]

Or we leave it as it is since we don't support platforms/compilers where real8 is not actually 8-byte reals...

I'd leave the flag as is but I am OK with change in docs to |USE_REAL8|ON, OFF|OFF|Whether to use double precision reals|

[letmaik]

Done. Let's target this feature for 4.1.1. So, do not merge until WRF-CMake 4.1 is released.

[dmey]

Agree -- added to 4.1.1 milestone to make it easier to track.

[dmey]

4.1.1 and 4.1.2 have been released -- do you still want to merge this in? Do we need to create releases? We can just wait for 4.2.0 otherwise...

[letmaik]

I'll merge it once CI passed. Please open a separate issue if you want to discuss updating to a new WRF version.