Travis CI should run a single script that runs individual tests - this also
lets developers run a single script and get easy-to-parse results.

POP now restores ALK and ALK_ALT_CO2, and that is reflected in an example input
file.

The --no_pause option for the build tests (bld_lib.py and bld_exe.py) means
those scripts run without the "Press [return] to continue..." prompt between
compilers. This is great for automated testing, and run_test_suite.sh uses this
new flag.

Also added a "make clean" test and "./bld_lib --no_pause" prior to the
"./bld_exe --no_pause" test.

Expect test failures since sphinx is not being loaded into Travis environment

Need sphinx, sphinx_rtd_theme (specifically 0.2.5b1, since that's what we are
currently using), and sphinxcontrib-bibtex

Also updated .travis.yml to pip install -r docs/py_requirements.txt and updated
run_test_suite.sh to only run Fortran tests if the executable builds correctly.

If bld_lib.py or bld_exe.py fails, then marbl_testing_class.py returns a status
of 1. (Similarly, if one of the stand-alone tests fail, the python returns 1).