"p_nom": pd.Series.min aggregation default reduces capacity
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## Checklist

- [x] I am using the current [`master`](https://github.com/pypsa/pypsa/tree/master) branch or the latest [release](https://github.com/pypsa/pypsa/releases). Please indicate:

## Describe the Bug

The default aggregation strategies are set in `networkclustering.py`
https://github.com/PyPSA/PyPSA/blob/daead70515d877fd273cc294ffa1734c218f2831/pypsa/networkclustering.py#L86-L98

We observed [here](https://github.com/pypsa-meets-africa/pypsa-africa/issues/431) the following behaviour using the PyPSA default aggregation strategies:

PyPSA-Africa: Powerplants in Nigeria **8GW**, after applying the aggregation strategy `p_nom: min` **5GW**, after fix `p_nom: sum` **8GW** 
PyPSA-Africa: Powerplants in Africa **121GW**, after applying the aggregation strategy `p_nom: min` **21GW**
PyPSA-Eur: Powerplant in Europe **476GW**, after applying the aggregation strategy `p_nom: min` **339GW**, after fix `p_nom: sum` **476GW**

I think the `"p_nom": pd.Series.sum` is an untended default setting introducing major capacity reductions. I would even argue that using `min` can be considered as bug. When clustering the powerplant capacities of e.g. 5 nodes, I want to have all 5 powerplants represented as `sum` in the resulting clustered node.  Therefore, I think we should make `"p_nom": pd.Series.sum` the default. 
diff --git a/pypsa/networkclustering.py b/pypsa/networkclustering.py
index 7ac9ea46..4acb1b47 100644
--- a/pypsa/networkclustering.py
+++ b/pypsa/networkclustering.py
@@ -86,7 +86,7 @@ def aggregategenerators(
     strategies = {
         "p_nom_max": pd.Series.min,
         "weight": pd.Series.sum,
-        "p_nom": pd.Series.min,
+        "p_nom": pd.Series.sum,
         "capital_cost": pd.Series.sum,
         "efficiency": pd.Series.mean,
         "ramp_limit_up": pd.Series.mean,
['pypsa/networkclustering.py']