Removed functionless make_source files.
Made arguments to Propagator object optional

If a user wants to use a Propagator object, we shouldn't make them jump
through all the hoops of calling the constructor with every argument, as
often it's unnecessary. Also moved the quark mass to make it non-
optional, as that's a pretty key piece of information.
Set effmass function in twopoint to use real valued correlator.