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
add SACPZ write support for Inventory (tested against one station served by IRIS SACPZ WS)
changelog
clarification for DIP header comment in SACPZ output
Response.get_paz() returns deepcopy now, much safer

additional minor fixes
Merge pull request #879 from megies/write_sacpz

add SACPZ write support for Inventory
Fix build errors.

One missed print statement and one minor flake8 warning.
Remove unnecessary plt.gca() calls.
Merge pull request #892 from mbyt/SPLINE

Replacing GPL spline code by LGPL spline code
docstrings
fix a bug introduced in 6a914e66d02, see #903
add degrees2kilometers helper routine (inverse of already existing
kilometers2degrees) + basic test