more complete testing in sasmodels

[pkienzle]

Current automated testing is only looking at models.

• Should use nosetest on all of sasmodels/*.py.

• Use --with-doctest in case any functions come with doc tests.

• There may be doctests in the sphinx tree, which can be discovered with --doctest-extension=rst.

• Automate tests of the examples.

• Examples should be pulled into the docs, possibly using bumps/doc/pylit.py to convert a runnable example into an rst file.