Refactor: presolving based on structural matrix

[fwitte]

The changes in this PR will be API breaking, therefore it points to the v0.8 branch.

This is becoming a major refactoring, not all things are finally decided

• All equations can now return a structure_matrix entry, which allows to identify linear dependencies in variables in the presolving.
• Linear dependent variables (i.e. a couple of pressure values) will automatically be represented by a singe variable, to which all the linear dependents point.
• The presolver will be able to identify, if users specified
  • more than one value in a set of linear dependent variables
  • have a circular specification in a set of linear dependent variables
  • potentially more things to follow
• The equations collected from connections, components and buses will be indexed. By this the user
  • can investigate, if an equation returns all zero entries for the jacobian
  • can see, which are the equations applied in the model
  • potentially more things to follow
• Placing derivatives in the Jacobian is simplified through wrapper functions

The changes will serve as basis for future changes

• The user can implement custom variables into their problem
• A structural analysis of the problem through a dependency matrix will be possible (are there any separated parts in the matrix, also see Solve of isolated parallel networks #612)
• Power connections (a new connection class, serving for non-material energy flow), with that explicit motor and generator components
• The starting value identification could be refactored to leverage a very similar idea as in the linear dependents finder of the presolving