Add alternative grid reinforcement methods (grid separation, switch balancing)

[batuhansanliii]

Description

Adds an alternative grid reinforcement framework that goes beyond the default "add a parallel
line" measure, implementing the additional methods requested in #377 (grid separation, switch
balancing, etc.) and a selection logic that picks a measure per situation.

Problem (see #377)

Default reinforcement can only add parallel lines, which produces unrealistic results (large
numbers of parallel lines) and offers no logic for choosing an appropriate measure per situation.

Solution

• New module edisgo/flex_opt/reinforce_grid_alternative.py:
  • reinforce_line_overloading_alternative(...) — orchestrator that selects a measure via
    add_method (relocate_circuit_breaker, add_station_at_half_length,
    split_feeder_at_half_length, add_same_type_of_parallel_line).
  • reinforce_lines_voltage_issues_alternative(...).
• New measures in edisgo/flex_opt/reinforce_measures.py:
  • add_station_at_half_length / add_station_at_2_3_length — grid separation (Netzauftrennung)
  • relocate_circuit_breaker — switch balancing (Lastflusssteuerung über Schaltzustände);
    relocates the open ring circuit breaker to the point of least load/generation difference
    between the two half-rings (consolidates the prototype from [Superseded by #340] Optimal relocation of circuit breakers #296).
  • split_feeder_at_half_length, split_feeder_at_2_3_length,
    add_same_type_of_parallel_line, add_same_type_parallel_line_voltage_issue.
• Cost extensions in edisgo/flex_opt/costs.py: cost_breakdown, per-cable-type costs,
  circuit-breaker and transformer costs; updated equipment-parameter CSVs and
  config_grid_expansion_default.cfg.
• Minor changes in tools/plots.py and network/topology.py.

Part of #377. Supersedes #296 (its commit b9036237 is fully contained here).

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change — see open points below (changed cost-function signatures, renamed
  _reinforce_station_overloading)

Current state / open points before merge

• Branch is CONFLICTING with dev and needs a rebase (dev moved substantially since Nov 2022).
• No tests yet for the seven new measures.
• No whatsnew entry; several docstrings are unfinished (e.g. "todo: docstring is to be updated").
• Potentially breaking: costs.line_expansion_costs / transformer_expansion_costs lost their
  default None argument, and _reinforce_station_overloading was renamed to _station_overloading
  — existing callers need to be checked.
• A selection logic that automatically picks the cheapest valid measure (rather than requiring an
  explicit add_method) is not yet implemented — see acceptance criteria in [FEATURE] Add grid reinforcement methods #377.

Checklist:

• New and adjusted code is formatted using the pre-commit hooks
• New and adjusted code includes type hinting now
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The Read the Docs documentation is compiling correctly
• If new packages are needed, I added them to setup.py etc.
• I have added new features to the corresponding whatsnew file