BUG : fixed addbackground function

The function had undefined variables and would not run with out
exception.

It also seems that it is used no where in the library and should
probably just be removed.
MNT/DOC : made addtheory function clearer

Let python do the un-packing for us.  Again, it looks like this function
is not actually used anywhere else in the code.
MNT/TST : updated import so test will run
Merge branch 'function_cleanup' of https://github.com/tacaswell/pymca into Specfit
Recover the functionality of several test methods.