Initial test coverage and refactoring

- New library directory structure
- Test runner executable
- Initial test coverage pattern
Changed function and variables names for clarity
Tests for set_coefficients passing

Created a new function that sets up the correct polynomial coefficients
when you tell it which color you are using. Tests are all passing.
Added helpful messages to exceptions
Modified set_coefficients to handle setting range

Rather than writing a separate function to handle setting the range min
and range max, I modified set_coefficients by renaming it set_constants
and having it return a tuple containing a list of coefficients, the min,
and the max of the validity range. set_constants is now the one-stop
shop for defining which polynomial to build.
Implemented validity check

Function validity_check makes sure that the color_value given is within
the valid range of the polynomial fit
Bolometric Correction implemented

Finished implementing the calculation of the bolometric correction, and
the bc_polynomial module has complete unit tets coverage.
Refactored luminosity code, tests in place

Refactored the code that actually calculates the luminosity and put it
under unit testing. I need to verify that I have complete test coverage,
and then this branch will be ready to merge into develop.
Forgot to add test to git.
Updated docstrings
Updated docstrings again
Added footer to test_luminosity.py
Merge branch 'test_coverage' into develop