Changes to the plotting of source time functions.
Transparent background for raydensity plot.
No more allowing unstable filters for source time functions.
Source time function now uses two lower order butterworth filters.

This in essence also results in a pretty sharp filter (the spectra are
just multiplied in the frequency domain) but with (hopefully) less
numerical issues.
Ignore plotting warnings like colormap overflows and what not.
Adapting CLI to the STF plotting changes.