Make summary variables an attribute of the model

[rtimms]

At the moment the simulation stores some summary variables, but these are hardcoded into solution. It would be nice if the list of summary variables were defined on the model so that different models could have different summary variables, or users could define their own list/add their own variables. Might need some checks (e.g. need to be scalars).

[valentinsulzer]

Yeah this is a good idea. Pretty easy, just make this list an attribute of the model? And have the others (minQ/maxQ, eSOH variables) hardcoded

[valentinsulzer]

When doing so, also need to check that every variable in the list is also a variable of the model