Add _apply method to VectorSpaceBasis and use it

vsb._apply(m) now sets vsb as a null space on m.
Add a MixedVectorSpaceBasis class

Representation of a null space for a mixed space operator where you only
care about the null spaces of the individual blocks.
Document MixedVectorSpaceBasis in solving interface
Add test of nullspace for mixed poisson problem
Merge pull request #182 from firedrakeproject/mixed-nullspace

Add ability to specify null spaces for mixed operators.