Compatibility with oemof.solph v0.5.1

[jnettels]

Version 0.5.1 of oemof.solph introduced the (experimental) feature of supporting multi-period investments: https://oemof-solph.readthedocs.io/en/stable/changelog.html#v0-5-1-august-31st-2023

The update changes the way certain objects with a time index are referenced, since the index now consists of a (periods, timesteps) tuple. This breaks the DHNx code in a couple of places.