Variable / scalar or LinearExpression / scalar is broken

[coroa]

Just saw this example:

https://github.com/prakaa/Battery-Optimisation-Benchmarking/blob/master/battery_optimisation_benchmarking/python/linopy.ipynb

With the following soc constraint definition (slightly cleaned)

    # intertemporal energy balance constraints
    intertemp_soc_lhs = (
        soc_mwh
        - soc_mwh.shift(datetime=1)
        - charge_mw * charge_eff * tau
        + discharge_mw / discharge_eff * tau
    ).isel(datetime=slice(1, None))
    intertemp_soc = m.add_constraints(
        intertemp_soc_lhs, "=", 0, "intemporal-energy-balance"
    )

where the operation discharge_mw / discharge_eff is bypassing the linopy definitions and directly dividing the variable labels, and does not even raise an error.

For correct operation needs to be realigned to:

    intertemp_soc_lhs = (
        soc_mwh
        - soc_mwh.shift(datetime=1)
        - charge_eff * tau * charge_mw
        + tau / discharge_eff * discharge_mw 
    ).isel(datetime=slice(1, None))

=> need implementations for __div__ and __truediv__ with a scalar on Variable and LinearExpression.

[FabianHofmann]

Indeed, this is again en example for unsafe operations. Easiest is to directly address it in #55 right?