Merge multi-precision support into main

[rouson]

This PR is will be ready merge either when gfortran's support for parameterized derived types (PDTs) has been fixed or we have decided to drop support for gfortran.

The develop branch that this PR will merge into main resulted from merging our two primary development branches:

1. The parallel branch added support for automatic parallelization of training via do concurrent.
2. The parameterized-derived-types branch added the ability to support both default-real and double precision inference without requiring recompiling.

The latter two branches have been deleted in order to clarify the path forward. Starting now, new work should branch off of develop.