Add parameters 'caption' and 'mask' in PlotEquiMap()

Hi @aho

Opening this issue to request the parameters 'caption' and 'mask' that are available in PlotRobinson() for PlotEquiMap(). In the case of 'caption', some special considerations might be necessary if PlotLayout() is used (maybe similar to 'toptitle').

Adding @nperez as she is also interested in this, please feel free to add any relevant information.

Cheers,

Victòria