Allow combining snapshot_clustering and skip_snapshots

[ClaraBuettner]

It is currently not possible to use snapshot_clustering and skip_snapshots in combination.
At least two things need to be changed.
First, the snapshot_weigthing is currently overwritten in skipp_snapshots, so the weighting of the previous snapshot_clustering is overwritten:

eTraGo/etrago/cluster/snapshot.py

Lines 833 to 835 in 94acf8d

	self.network.snapshot_weightings["objective"] = n_skip
	self.network.snapshot_weightings["stores"] = n_skip
	self.network.snapshot_weightings["generators"] = n_skip

In addition, the constraints for the soc need to be updated. The soc_intra-rule for example wants to refer to the previous timestep using a fixed offset of one hour which results in errors when that one is skipped. It might be also needed to consider the number of skipped snapshots in these constraints:

eTraGo/etrago/tools/constraints.py

Lines 1759 to 1768 in 94acf8d

	else:
	expr = m.state_of_charge_intra[s, h] == m.state_of_charge_intra[
	s, h - pd.DateOffset(hours=1)
	] * (1 - network.storage_units.at[s, "standing_loss"]) - (
	m.storage_p_dispatch[s, h - pd.DateOffset(hours=1)]
	/ network.storage_units.at[s, "efficiency_dispatch"]
	- network.storage_units.at[s, "efficiency_store"]
	* m.storage_p_store[s, h - pd.DateOffset(hours=1)]
	)
	return expr

For the next release, I will not fix it but implement a check that makes sure that those methods are not combined.