trivial helpstring change
Numerous improvement to the event plotting.

* Plotting just a single event now supported for all projections/color
  encodings.
* Color encoding now defaults to the event depth.
* New colormap for the event depth encoding.
* Now uses the preferred magnitude and origin if available.
* Colorbar hidden for single events.
* Adjustments to the map colors for better visibility of the events.
* Some bugfixes and workarounds.