Rearanged Snufflers help menu
so that help ofsnufflings without a panel can be accessed
Merge branch 'master' into snuffling_help_menu
added space partitioning interpolation algorithm
wip (still with InterPhase class)
moment_tensor: added function to calculate angle between two MTs
cake: let some phases work for zero distance
Merge branch 'master' into snuffling_help_menu
Merge branch 'snuffling_help_menu' of git://github.com/HerrMuellerluedenscheid/pyrocko into marius2
snufflings: fixed problem with multiple menu entries in help menu

After each reload of a snuffling there was +1 help menu entry for that
snuffling.
snufflings: allow snuffling docs to optionally be written in HTML

If the snuffling __doc__ text starts with '<html>' it is assumed to be
html. Otherwise markdown is assumed.
snufflings: fixed problem with snufflings without __doc__
snuffler: help windows are now non-volatile
snufflings: snuffling code is now shown in help
mseed_ext: corrected type of psamples argument
docs: added an example snuffling to demonstrate how markers are created