New interpolate() method for Stream and Trace objects.

Currently only wraps scipy.interpolate.interp1d but that already offers
a bunch of options.
Added weighted average slopes interpolation scheme.
Adding test data.
Refactored file name
More testing and a bugfix
Added test case for the various interpolation settings.

No extrapolation can be performed so the new sample points
must be fully contained in the old ones. Furthermore the
sanity of the npts and starttime settings has been tested.
Added sanity checks for non-positive sample intervals.
Added mock test for streams.
Protection against inter-sample extrema
Avoiding division by zero.
Using a different method for spline interpolation.

The old one used an obscene amount of memory.
Using a custom interpolation scheme.

The piecewise polynomial in scipy is too memory intensive.
Hermite interpolation now performed in C. Much faster.
Moved files to sac tests folder
Bugfix in the C code.
Properly documenting C code.

Also added some small changes to the required arguments.
Changelog.
Changed  to
pep8.
Moving declaration out of for loop as gcc complained.