[FEATURE] Improve default voltage-issue and LV grid separation reinforcement measures

[joda9]

Motivation / Problem

The existing (default) grid reinforcement measures for voltage issues and for highly
overloaded LV grids have a few shortcomings:

1. Voltage issues in LV feeders are always solved by splitting the feeder at 2/3 of its
   length, even when the actual cause is one or more undersized cables in the feeder.
   Exchanging the undersized cable for a standard cable would be the cheaper / more realistic
   measure in those cases.
2. LV grid separation always connects the split point to the (new) station with the standard
   line type, even if the original line was larger. This can under-dimension the connecting
   line.
3. New MV/LV transformers created during LV grid separation get type_info = None, which
   breaks the transformer cost calculation downstream.

This is related to the broader effort in #377 (add/improve grid reinforcement methods) and to
the voltage-band discussion in #350, but concerns the existing default measures rather than
the alternative method framework.

Proposed solution

• In reinforce_lines_voltage_issues (edisgo/flex_opt/reinforce_measures.py):
  • Use grid.assign_grid_feeder() / the grid_feeder column to group critical buses per feeder.
  • For LV feeders, first exchange undersized cables (NAYY 4x1x120/95/50/35, excluding
    house-connection end cables) by the standard cable, adding parallel cables when the standard
    cable would reduce s_nom. Only if no cable was exchanged, fall back to the 2/3 split.
  • Extract the 2/3-split logic into a reusable function
    split_feeder_at_given_length(..., disconnect_length=2/3).
• Add a helper get_standard_line() to deduplicate the standard-line config lookup.
• separate_lv_grid / run_separate_lv_grids / enhanced_reinforce_grid: add a
  use_standard_line_type parameter (when False, keep the original line type and number of
  parallel lines for the connecting line).
• Fix the new transformer's type_info to the standard transformer name (cost-calculation fix).

Affected files

• edisgo/flex_opt/reinforce_measures.py
• edisgo/flex_opt/reinforce_grid.py
• edisgo/network/topology.py

Acceptance criteria

• Undersized LV cables are upgraded before falling back to the 2/3 feeder split
• use_standard_line_type option works for LV grid separation
• New MV/LV transformers carry a valid type_info and are costed correctly
• Tests cover the new behaviour (the existing test_reinforce_lines_voltage_issues is extended)
• whatsnew entry added

Implemented by

• PR Improve default voltage-issue and LV grid separation reinforcement measures #391

Related

• [FEATURE] Add grid reinforcement methods #377 (add grid reinforcement methods)
• [BUG] Grid reinforcement using only 10% criterion not correct #350 (combined / voltage-band reinforcement)