Add namelist and streams filename parameters to mpas_init

[svahl991]

This alters the signature of the mpas_init subroutine in mpas_subdriver.F, adding two optional parameters that allow the caller to pass in filenames for the namelist and streams file. This allows the caller to specify different files to be used, other than namelist.atmosphere and streams.atmosphere in the working directory. This functionality is critical for several use-cases in MPAS-JEDI, including the ConvertState application, which interpolates data between two MPAS meshes of different resolutions.

[svahl991]

This change is required for JCSDA-internal/mpas-jedi#360.

[junmeiban]

Already tested. It works with my case https://github.com/JCSDA-internal/mpas-jedi/pull/495. Thanks @svahl991 .

[jjguerrette]

Looks good. Please add this PR to MPAS-Dev#789.

[svahl991]

Please add this PR to MPAS-Dev#789.

Done.

[liujake]

This looks good. But should we just use stream/namelistFileName instead of stream/namelistFileParam? That is more readable to me.

[svahl991]

But should we just use stream/namelistFileName instead of stream/namelistFileParam? That is more readable to me.

Well, I just merged this, although we can still change the variable name. But my thinking in putting Param at the end, which I agree is a little ugly, was to make a clear distinction between the filename variables passed in as parameters, and the already existing variables namelistFile and streamsFile in the subroutine. If we used your suggestion, we would have two variables, one named namelistFile and one named namelistFileName, and to me it is then unclear what the difference is between them. That was my thinking anyway.

[liujake]

Fine with me.