unit test

[kslong]

There are two small problems with the unit_test implementation that should be corrected

• Currently the Makefile for unit tests has to be updated if a change is made to the c files needed to compile sirocdo. This should not be necessary. The unit test make file should get the information that a change has been made by importing it from the main source makefile
• When I was experimenting with tightening the requirements for calculating ne in the ionization routines, the unit_tests failed because a variable FRACTIONAL_ERROR was changed. FRACTIONAL_ERROR should be defined independently in the unit tests, so that the unit tests are more independent of python.h

[jhmatthews]

On point one, i think we can do this with these lines:

include $(SIROCCO)/source/Makefile 
SIROCCO_SOURCES = $(sirocco_source) get_models.c

I don't know why we need the get_models.c separately, but there's a comment about this in the main Makefile.

Point two: yes, we can change this to ERROR_TOLERANCE or something, defined only in tests.