Version Checks (indicate both or one)

• I have confirmed this bug exists on the lastest release of PyPSA.

• I have confirmed this bug exists on the current master branch of PyPSA.

Issue Description

I tried to add a custom variable to my model. This resulted in an error when trying to solve the model.

Here is the resulting traceback:

Traceback (most recent call last):
  File "/main.py", line 13, in <module>
    n.optimize.solve_model(assign_all_duals=True)
  File "/pypsa/optimization/optimize.py", line 598, in solve_model
    self._n.optimize.assign_solution()
  File "/pypsa/optimization/optimize.py", line 620, in assign_solution
    c = n.c[_c_name]
  File "/pypsa/components/store.py", line 134, in __getitem__
    return super().__getitem__(item)
KeyError: 'custom'

Reproducible Example

import pypsa

# Create minimal network
n = pypsa.Network()
n.add("Bus", "bus")
n.add("Load", "load", bus="bus", p_set=1)
n.add("Generator", "generator", bus="bus", p_nom=1, marginal_cost=1)

# create custom variable
n.optimize.create_model()
m = n.model
m.add_variables(name="custom-var", coords=[n.snapshots])

# solve model
n.optimize.solve_model()

Expected Behavior

Custom variables with arbitrary name should not break the optimization.

Installed Versions