Update documentation; improve behaviour on no data

[rod-glover]

• Resolves #207

• Adds documentation for local preferences.

• Adds Period (climate regime) and Global Warming to local preferences.

• Improves behaviour on no data:

  • Updates map message.
  • More significantly, clears Data from map pointer items when DV is changed. This prevents a lot of traffic to the backend when a user is just examining maps after having clicked a map location. It also ensures that the map-click location is always based on the current map, whose rasters--and hence click locations-- can differ.
  • Also fixes callback errors introduced by allowing no-data selections.