Prepend environment variables set by METplus with identifier

[georgemccabe]

Some wrappers set environment variables so they can be read by the MET config files. We should add an identifier to the beginning of each of these to ensure we don't overwrite an environment variable that is used by the user and to better organize these variables. MP_ is a common prefix for MPI environment variables, so we can't use that. Jim suggested MPE_ (METplus environment). Could we just use MET_ or would that conflict with MET environment variables? These changes will have to be made in the wrappers that set environment variables and all MET config files in the METplus repository that use these variables.

[georgemccabe]

For v4.0.0, we have enhanced the wrappers to set environment variables that start with METPLUS_ to be referenced in wrapped MET config files. This was implemented in multiple pull requests associated with this issue:
#768