Ast improvements

[thesamovar]

OK I think I've done all the work I wanted on this branch, written tests and documentation - it's ready to go!

[coveralls]

Coverage remained the same when pulling 99d478c on ast_improvements into d3d51bd on master.

[coveralls]

Coverage remained the same when pulling c55c122 on ast_improvements into d3d51bd on master.

[coveralls]

Coverage remained the same when pulling 188c583 on ast_improvements into d3d51bd on master.

[mstimberg]

I fixed a bug in the handling of "not" and worked around a Python 3 issue (arguments don't have an id attribute anymore). I also switched the unit checking to use the new parse_expression_unit function and adapted it to a namespace, specifiers signature. The function support is also there, It works automatically when you use the function wrappers that are used for all the default mathematical functions. I did not implement it for the check_units decorator yet, though. The exponent handling for the power operator also needed some changes for Python 3 (apparently, -0.5 is no longer parsed as a number, but as the unary minus operator applied to 0.5) -- the good thing about this work is that I used the opportunity to support constant, scalar variable values.

I'm only waiting for the current travis run to finish, then I'll merge.

[mstimberg]

That's odd, github thinks the travis build failed but on the travis page, it completed without error. Either way, merging now.