Working on issue #15

Cleaned up the wall API and tests pass.
One more change related to #15 to improve consistency
OK, hopefully the last commit related to consistency and #15 for wall API
Fixed up consistency issues related to #15 for melds
Finishing up changes for issue #15
Adding some final documentation

This closes #15.
Added API for specifying type constraints discussed in #2

I don't enforce anything yet.
Lots of excellent improvements

Added support for type constraints on columns in tables (and tests to test it).  This
closes #2.

Added tests for non-float types (int, bool and str).  This required me to deal with
places where flaots were assumed (e.g. scaling of alias variables).  The tests now
exercise a wide enough range of conditions that I'm convinced the files can be
quite heterogenous.