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.
pep8
typos and py3k
x axis inputs must be strictly increasing

thx to @QuLogic for pointing that out
beautifying
Merge pull request #892 from mbyt/SPLINE

Replacing GPL spline code by LGPL spline code