Class VolumeMapper

      object --+    
               |    
GeomAnalyzerBase --+
                   |
                  VolumeMapper

This class takes the coordinates of provided particles and maps them onto
a discretized grid using kernel density estimation as implemented in scipy.
The returned occupancy object is a volumetric np.array which contains the
probability density of the selected particles.

Note: The trajectory provided for this method should already be pre-aligned
      onto the selection of interest.  For example, to calculate the
      water occupancy around a protein, the provided trajectory should
      already contain the necessary transformations on the protein.


Basic usage:
    ana = VolumeMapper(cms_model, 'mol.num 1')
    results = analyze(tr, ana)