Replacing GPL spline code by LGPL spline code

Now using the LGPL SPLINE library
http://people.sc.fsu.edu/~jburkardt/c_src/spline/spline.html

Adding an extra test, as the current obspy tests do not seem
to cover the interpolation part.

See #827.
Ah, the original `evr_spline` accepts both, but the new `spline_cubic_set` does not. The input must be strictly increasing.