Make pn_bar optional in create_junction

[jkisse]

Currently, the create_junction function requires a value for pn_bar. This is used as the initial value for the Newton-Raphson solver. Apart from that, pn_bar does not have any further purpose. Originally, the argument had been introduced in close analogy to pandapower, where vn_kv is important for the calculated per-unit voltages.

pn_bar can be misleading for users who are used to other pipe simulation software, since it can be confused with maximum operating pressure (MOP) or a "pressure-fixed node" which are more established input arguments for junctions.

For better clarity, pn_bar can be renamed to p_init_bar.
It could also be changed to an optional argument with None as default value. Then, the initial pressure of the junctions without provided p_init_bar should be estimated based on the external grids and pressure controllers.
If there are more than one pre-defined junction pressures, the initial pressure value could be based on their averages. For more complicated grid topologies, the users can be asked to make use of the p_init_bar argument and provide reasonable starting values.

Please share your view on this :-)