Add supply for different fluid property back ends

[fwitte]

• thermosteam
• custom equations
• ...

[fwitte]

Update on the structure of this:

• cycles in the network are detected
  • within one cycle the fluid has to be specified on some connection, which in will also hold the information on the fluid property back end.
  • All connections within one cycle will use the same fluid property library
  • All connections within one cycle will access the same fluid property object
  • For that reason the fluid composition is only variable within a cycle if specific components (combustion, merging, …) are part of that cycle
• each connection can turn on and off its variables, following the fluid composition variability
• Components get reworked for checking which connections have what variables
• Component variables will get a proper place in the matrix

[fwitte]

Closed as completed via #384