Improve testing

(With contributions from @tlycken) There is lots of room for improvement in our test suite. Here are some areas that are in need of improvement:
- Vector-valued indexing (ref #24, #54). Test that indexing with arrays, ranges, linranges and linspaces yield the expected results. Test `Colon` indexing with `NoInterp` dimensions.
- Exact polynomials (see e.g. discussion in #90). A B-spline of degree n can be tested by interpolating data from a polynomial of the same degree, yielding results that should be exact down to the last ULP. We should test this.
- Boundary conditions: We don't test today that e.g. an interpolant with the `Flat` boundary condition really has a gradient tending to 0 toward the boundary.