This implements a change @wkerzendorf and I had tossed around some time
back of eliminating the need to provide a parameter's name twice when
defining the Parameter attributes on a Model subclass.

In other words, rather than having to write
``amplitude = Parameter('amplitude')`` one should be able to write just
``ampltiude = Parameter()`` and have the name automatically taken from
the class attribute (in fact, it is required that they be the same or
else other parts of the code will break--this requirement has now been
made explicit with a check in _ModelMeta.__new__).

This might be considered very "magic", but it's not the only aspect of
Parameter that's magical to begin with.  It also makes the code I think
terser and easier to read.
Merge pull request #1786 from embray/parameter-names

Change parameter definition to not require stating the name twice