Make PatchActionConvert reuse old cell processes

[allison-li-1016]

The PatchActionConvert class represents mutation of one cell type to another (e.g. a healthy tissue cell mutating into a cancer cell). Currently it is implemented by removing the current cell, making a new cell of the different cell type, and inserting the new cell into the simulation at the same location of the old cell.

Instantiating a new process for the new cell initializes all internal amounts at the initial concentration parameter values, which does not represent the current state of the mutated cell. Instead of creating a new process for the new cell, we want to re-use the same processes from the old cell to accurately represent the internal amounts of metabolites and signaling molecules.

[cainja]

This is a tricky one, maybe @jessicasyu will have some thoughts because I've never really used this action -

In order to update the module from the old cell, the modules would need to be the same.
(1) would there ever be a use case that the cell conversion that's happening would be from a population using one module to another using a different module (i.e. one uses simple and the other uses complex)
(2) have we used this in published simulations? I have only inserted cell populations and have not tried to convert them. it seems strange to only grab the cell at the center of the simulation if there is one...

[jessicasyu]

@cainja The goal of this action is to capture a healthy tissue cell "mutating" into a cancerous one sometime during the simulation.

1. I can't think of a use case, but the setup file does allow for different modules versions for each population, so you would technically need to support it
2. We have not used it in published simulations