Merge pull request #1789 from jedwards4b/srt_compiler_option

Add a --compiler and --mpilib options to scripts_regression_tests.py
to facilitate more testing with non-default compilers and mpilib's.

Test suite: scripts_regression_tests.py --compiler pgi on yellowstone + scripts_regression_tests.py

scripts_regression_tests.py --compiler gnu --mpilib openmpi on cheyenne
Test baseline:
Test namelist changes:
Test status: bit for bit
Addresses #1770

User interface changes?: Yes scripts_regression_tests.py now has --compiler and --mpilibs option