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

When setting a p_set time series in the storage unit component and running an optimization, the operation of the storage unit remain unconstrained. while you can fix p_discharge and p_store individually, the combination is much more intuitive and aligned with the other components.

I realize now, that this could also be a feature request, depending on how you see it.

Reproducible Example

import pypsa

n = pypsa.Network()
n.set_snapshots(range(4))

n.add("Bus", "bus")
n.add("Generator", "gen", bus="bus", control="Slack", p_nom=100)
n.add("Load", "load", bus="bus", p_set=[20, 30, 25, 35])

n.add(
   "StorageUnit",
   "storage",
    bus="bus",
    p_nom=50,
    max_hours=2,
    p_set=[10, -15, 5, -10],  # positive=discharge, negative=charge
)

n.optimize()

Expected Behavior

In order to be consistent with the other components p_set would need to fix combined p_dispatch - p_store.

Installed Versions