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.
To my understanding for t (original x-axis) increasing or decreasing values are allowed, e.g. [0, 1, 2, 3, 4, 5] or [5, 4, 3, 2, 1, 0]. Either of both `||` shall be allowed.