Improved usability of Makefiles

Replaced mastermake with a makefile, which calls the lower level make
files.  This allows systems that cannot update their default compiler
to install davitpy without manually changing all of the lower level
model makefiles.

Lower level makefiles (iri, hwm, tsyganenko, and raydarn) and the
README were also updated to be compatible with this change.

Excess spaces were removed from MPIutils.f90 since I opened it to see
what it did.