Fix Function call with Point, as reported by Jan Blechta
  -- add unit test for Function eval
  -- fix faulty eval test in Expression
Fix an Expression unit test.
Some VectorSpaceBasis interface fixes and add is_orthogonal function.
Use VectorSpaceBasis in PETScPreconditioner and change VectorSpaceBasis::size to VectorSpaceBasis::dim.
const fix for VectorSpaceBasis.
Add another fix for a faulty unit test of Function::eval.
Merge branch 'fix-issue-68'