Break metric, angular, through-vision and isovist analyses out of pointmap

[pklampros]

Isovist analysis is carried out using the currently visible drawing layers, hence for this to work the interface had to be extended to also accept the MetaGraph. The other solutions I can think of is to make a separate interface for modules that want to use the MetaGraph, or for the isovist analysis to use the drawing layers attached to the pointmap itself

Changed to make the isovist analysis take the drawing layers from the pointmap itself