Add a virus perturbation

[Bertinus]

When perturbed, add a NodeSet and EdgeSet to the CellPop object, which would correspond to the proteins of the pathogen and their interactions with human proteins.

Check the feasibility first. If too complicated, drop this idea.

Rely on the protein-protein interactions between covid and human proteins. https://github.com/RECOVERcoalition/Reservoir#covid

[Bertinus]

Check if we could have some logic inside the CellPop object to "infect the cell" with some external graph.

Note that if the shape of the cell state changes, we will need to update all the pointers to this state (in the NodeSet objects)

[Bertinus]

Investigate if we could infect just some of the cells instead of the whole population