Describe the bug
The Cell class mesa/discrete_space/cell.py maintains redundant state using both cell.empty and cell.is_empty.
This leads to:

• Possible state inconsistency cell.empty != cell.is_empty
• Unnecessary O(n) list copying during remove_agent()
• Use of a generic Exception when cell capacity is exceeded

Expected behavior

• Cell emptiness should have a single source of truth
• Checking whether a cell is empty should not copy the agent list
• Capacity overflow should raise a specific exception type

To Reproduce

from mesa import Model
from mesa.discrete_space import OrthogonalMooreGrid
from mesa.discrete_space.cell_agent import CellAgent

model = Model()
grid = OrthogonalMooreGrid((3, 3), random=model.random)
cell = grid[(0, 0)]

cell._agents.append(CellAgent(model))  # bypass API

assert cell.is_empty 
assert cell.empty == True # inconsistent state

Additional context

• remove_agent() triggers self.empty = self.is_empty
• is_empty → agents → _agents.copy() causes O(n) overhead
• Adding beyond capacity raises generic Exception

@EwoutH @quaquel I would like to know you thoughts on this particular issue. Can I raise a PR resolving the issue.