Improve testing

[timholy]

(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.

[tomasaschan]

Referencing #24 here too; it should be closed when we fix this.

[tomasaschan]

@timholy I took the liberty of fleshing out the body of this issue a little :)

[timholy]

Very good, thanks!