Fixed import error in a sparse test

Minor stuff (PEP8 and typos mostly)

The rephrasing is to avoid using 'type' for class name, given that
'type' has a special meaning in Theano.

Add "filter_variable" mechanism in Type

Jacobian/Hessian

If one of the "then" elements is a TensorType, and the corresponding
element in the "else" branch is a compatible type, or vice versa,
convert it into TensorType, so both have the same Type, and the
output Type can be determined.

This should fix a problem reported by Ian.
[Commit amended to fix problem spotted by Fred]

It should work similarly to filter(), but on symbolic variables,
returning equivalent variables of the current Type if they are
compatible.

Fixed import crash in optimizations

CCW#37: REVIEW/TEST/DOC theano/sparse/sandbox/sp.py:RemoveO

CCW#37: modified theano.config.mode value under which assertion of application of the optimization is done.

Test using None in slices in local_subtensor_merge