Ensure conda environment covers what is needed for METplus

As METplus will be run as part of CSET, we need CSET's environment to provide everything that METplus needs.

@JorgeBornemann when you talk about the METplus environment, does that just need to be what is required to run METplus, or is that everything required to compile and build it too?

_Issue transferred from ._

The issue does not name a file, test, or entry point. First clarify whether CSET must support only running METplus or also compiling and building it, then identify the existing environment definition and METplus requirements. Done means the CSET environment provides the agreed scope and can be verified for a METplus run.

Written by the indexing model from the issue text.