Revise Power Flow Calculation

[antonplietzsch]

I think the power flow calculation needs to be revised. The main issue is that for every new node that is implemented, one also has to implement a corresponding method for converting the node into a PowerModels-Dict-entry in the power_flow.jl file. Additionally, the parameter limits for the power flow calculation are hard coded in these functions and set to rather arbitrary values at the moment.

Here are some ideas for what we could do:

1. The make_bus_ac! function dispatches on the PowerDynamics node type. However, the main difference in these methods is basically to which PowerModels bus type each PowerDynamics node type is converted (generator nodes become PV-buses, load nodes become PQ-buses, etc.). I think it should be possible to reduce a lot of redundant code by somehow directly specifying the corresponding PowerModels bus type in the definition of each PowerDynamics node.
2. Also the parameter limits for the power flow calculation could be set as node parameters with a default value such that they are no longer hard coded in some hidden place.