DifShell and DifBuffer implementation

Created by: asiaszmek

This is based on the work of Subhasis Ray and the genesis implementation of difbuffers and difshells.

I tested this implementation by running a simple model of a one dendritic segment with one Ca channel and a bunch of different calcium scenarios. I ran this model with one difshell and the Ca channel; two and three difshells with the Ca channel; with two difshells, one buffer and the Ca channel; with a difshell and a pump and the Ca channel; one difshell and one difbuffer without the Ca channel but out of equilibrium, three difshells and one difbuffer without the Ca channel and out of equilibrium. In cases of Ca influx I compared model's output to results given by genesis implementation of the same model. In out of equilibrium cases I compared it to theoretical solutions.