[FEATURE] Add grid reinforcement methods

[khelfen]

Motivation / Problem

Grid reinforcement is currently very simplified and can produce unreasonable results — e.g. a large
number of parallel lines — because only a limited set of measures exists and there is no logic for
choosing the appropriate measure per situation.

This issue is part of #15.

Proposed solution

• Add further reinforcement methods (see dena Verteilnetzstudie, p. 114):
  • grid separation (Netzauftrennung)
  • switch balancing (Lastflusssteuerung über Schaltzustände)
  • further measures (e.g. ordering of line vs. transformer upgrades, voltage regulation)
• Add a methodology to decide when which reinforcement method is applied (selection logic / cost-benefit ordering) instead of always defaulting to parallel lines

Affected files

• edisgo/flex_opt/reinforce_grid.py
• edisgo/flex_opt/reinforce_measures.py
• edisgo/edisgo.py (EDisGo.reinforce() entry point / parameters)

Acceptance criteria

• At least grid separation and switch balancing are implemented and selectable
• A documented selection logic picks the method based on the constraint type and avoids unreasonable parallel-line results
• Tests cover each new method
• New methods and selection logic are documented

References / related

• Parent: Implement grid reinforcement method #15
• dena Verteilnetzstudie (link above)