* Add support for IBM threaded compilers, xl*_r

Added new compiler class, xl_r; added default flags to the compilers.yaml file.

* Add cppflags to the set of default flags to be added to the compilers stanza in compiler.yaml.

These flags are optional. Only defined flags will be listed in the compilers.yaml file.

* Fix scripting warnings revealed by flake8.

Updated __init__.py and xl_r.py to conform with flake8 rules.

* Add justification to the definition of the XL default compiler flags.