TST: bsplines/periodic: tst vs full matrices.

Numerical stability is way too bad at the moment,
the feature is not usable.
API: bsplines: add some intelligence to make_interp_spline

Default the spline order to cubics, provide a sensible
default for the knot vector
TST: bsplines: make_periodic spline w/ random coefficients

This fails and needs to be fixed.
TST: tweak the test for the ppform of a bspline

pp-form and b-form only agree on the base interval
t[k]..t[-k-1].