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.
setting this #if 0 to #if 1 introduces an error of a factor of 100 to all interpolated values.

However, this does not lead to any failure in the obspy test-suite. Therefore, I introduced test_evalresp_spline.