Analysis to identify lipid - non-lipid neighbours

[p-j-smith]

Is your feature request related to a problem? Please describe.
lipyphilic.lib.Neighbours currently finds all lipid-lipid neighbours in a membrane. However, it would be useful to be able to find lipid - non-lipid neighbours, such as lipid-water or lipid-drug. This would enable the calculation of the local lipid environemnt of permeants as they diffuse through a membrane.

Describe the solution you'd like
Add an optional neighbour_sel keyword to lipyphilic.lib.Neighbours. If provided, neighbours between residues in lipid_sel and neighbour_sel would be identified. The default would be None, in which case lipid-lipid neighbours would be found.

Describe alternatives you've considered
Create a new class for the lipid - non-lipid neighbour analysis. This might be easier than modifying the largest_cluster and count_neighbours methods to handle both lipid and non-lipid neighbours.

Additional context
Ghysels et al. calculated the local lipid environment of water molecules as they permeate through bilayers.