Describe the bug
When instantiating a grid object (e.g., OrthogonalVonNeumannGrid) there is a capacity parameter that ostensibly should limit the number of agents that each cell in that grid can accommodate. But this does not appear to be used, at least in many of the places where this limit could be exceeded.

Expected behavior
I expected an "overfull; move_to operation denied" error to arise when I did this:

m = Model()
grid = OrthogonalVonNeumannGrid((2,2),torus=False, capacity=1)
some_cell = grid.all_cells.cells[0]
bob = CellAgent(m)
julie = CellAgent(m)
bob.move_to(some_cell)
julie.move_to(some_cell)     <-- I expected this to throw an error.
print(some_cell)

output:
Cell((0, 0), [<mesa.discrete_space.cell_agent.CellAgent object at 0x7787172616d0>, 
<mesa.discrete_space.cell_agent.CellAgent object at 0x7787172611d0>])

Instead, the cell happily accepts two agents, even though it is in a grid whose capacity is set to 1.

To Reproduce
See code in previous section.

Additional context
If capacity isn't enforced, then why does it exist? One would think its sole purpose would be to act as a guardrail against attempts like in that sample code.

If this behavior is intended, then at the very least the docstring should have some "just kidding, it doesn't actually enforce this" language. At present it reads:

Type:           OrthogonalVonNeumannGrid
...
Args:
   ...
    capacity: capacity of the grid cell
  ...