fix p_nom_min for renewable generators in myopic configuration

[martavp]

In the overnight configuration, p_nom_min for renewable generators is set based on existing historical capacities. This is fine.

In the myopic configuration, when setting p_nom_max for renewable generators, the existing capacities are enforced over the maximum potential based on available land. The existing capacities have been written by prepare_network.py as p_nom_min in every expandable renewable generator). This is not correct.

A practical example, let's say that the existing solar capacity before 2020 in Spain is 5000 MW, and we want to run myopic for [2020,2030]
prepare_network.py creates two prenetworks with
generator.p_nom_min['ES solar-2020']=5000
generator.p_nom_min['ES solar-2030']=5000
but during the myopic run for 2020, the optimization yields generator.p_nom_opt['ES solar-2020']=maximum capacity based on available land.

Then, what should happen is that generator.p_nom_min['ES solar-2030']=0

This is not what it does currently.

Checklist

• [x ] I am using the current master branch.
• [x ] I am running on an up-to-date pypsa-eur environment. Update via conda env update -f envs/environment.yaml.

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