Add VectorSpaceBasis::is_nullspace function

[Prof Garth Wells]

Add the function

bool VectorSpaceBasis::is_nullspace(const GenericMatrix& A);

to test if vector space is the null space of A. PETSc provides a function to do the test.

[Prof Garth Wells]

Add free function in_nullspace to check that a VectorSpaceBasis is in the nullspace of an operator.

Fixes Issue #90.

→ <<cset 43a202fe77c8>>

[Prof Garth Wells]

Removing milestone: 1.5 (automated comment)