Add support for building MPAS on titan with cray and pgi

[douglasjacobsen]

This adds build targets for PGI and Cray compilers on titan.

[douglasjacobsen]

This was provided as part of https://github.com/MPAS-Dev/MPAS-Release/pull/14

[mgduda]

This looks almost identical to the pgi-nersc target.

[douglasjacobsen]

Yes, this one does look identical. We can probably remove it from the PR.

[mgduda]

I think we may be on the verge of a slippery slope when it comes to build targets; if we can determine that the new targets are in fact compatible with existing targets, it might be preferable to use the new targets as synonyms for the existing targets, and to perhaps generalize the existing targets.

[douglasjacobsen]

I know at least the titan-cray build target is not compatible with previous build targets, or at least the modifications are required for titan at a minimum.

I think if the modifications are also compatible with nersc machines, it might be preferable to merge the two together, and just update the modifications, but I don't know if the flags work on nersc machines as well and I'm not sure if there will be time to test this prior to our release.

[douglasjacobsen]

I removed the titan-pgi build target. But I'd still like the titan-cray build target for now.

In the future we can look at consolidating these build targets together, but that won't happen prior to v3.0.