Addition of new visibility object which allows anisotropy

[mwiet]

Within glass.observations, the addition of a new visibility object, which allows anisotropic and non-binary visibility HEALPix maps which vary with galaxy population/tomographic bin/shell, would make the implementation of variable observational depth possible.

The object should be flexible enough to allow independent visibility maps for each galaxy population/tomographic bin/shell, while also allowing the option of providing a single HEALPix map of an independent variable which traces variable depth plus arrays of functions which characterise the variation with galaxy population/tomographic bin/shell. The latter is particularly useful as it allows for memory efficient modelling of varying depth.