ENH: bsplines: periodic/antiperiodic interpolators

Construct interpolators where derivatives at the edges are constrained
by a single linear relation, y(x_left, j) = alpha * y(x_right, j)
for j in range(1, k). This way we can have both periodic interpolators
(alpha=1), and antiperiodic ones (alpha=-1).
API: bsplines: make_interp_* helpers return tck

Given t and k, compute c, return all three together.