Behavior of subclassing groups differs between net and cell.

[michaeldeistler]

If one subclasses groups of net then one needs to use the global index of the cell (i.e. in the entire network):

net = jx.Network([cell for _ in range(5)])
net.cell(3).add_to_group("exc")
net.exc.cell(3).set(...)

Contrary to that, for cell, one has to use the local index of the branch within the group:

cell = jx.Cell(Branch, [-1, 0, 0])
cell.branch(2).add_to_group("exc")
cell.exc.branch(0).set(...)

[michaeldeistler]

I think this can be closed, right @jnsbck ?

[jnsbck]

Should have been addressed in #447, will add a test for this in #453