Apply per-timeslice link limits as per-snapshot p_max_pu/p_min_pu series

[nick-gorman]

Functionality for adding per timeslice transmission limits to the pypsa network in the pypsa_build module.

Pypsa friendly tables of the form (abreviated data):

link_timeslice_limits:
    name            attribute  timeslice        value
    CQ-NQ_existing  p_max_pu   qld_peak_demand  0.857
    CQ-NQ_existing  p_max_pu   ,                1.0     # fallback
    CQ-NQ_existing  p_min_pu   ,                -0.714  # fallback only

timeslice_snapshots:
    timeslice_id     investment_periods  snapshots
    qld_peak_demand  2025                2025-01-13 12:00

Get transformed and inserted into the Pypsa model such that:

network.links_t.p_max_pu:
    investment_periods  snapshots         CQ-NQ_existing
    2025                2025-01-13 12:00  0.857
    2025                2025-01-15 12:00  1.0
        
network.links_t.p_min_pu:
    investment_periods  snapshots         CQ-NQ_existing
    2025                2025-01-13 12:00  -0.714
    2025                2025-01-15 12:00  -0.714

A few points:

• The implemention is defensive such that if an non-extendable link doesn't have a limit value for each snapshot, from an explicitly named timeslice or a NaN fallback, then a error is raised. Might not be needed when validation is fully in-place, but it was an easy way to satisfy myself about the correctedness for now.
• Also reached into the translator and changed how zero-capacity new corridors are treated, now they aren't a special case and just get timeslice limits set to zero.
• One known gap: a limits table whose timeslice column is blank in every row comes back from CSV as float64 and can't be merged onto an empty timeslice_snapshots. That's a general CSV-round-trip typing problem (Column dtypes change on CSV round-trip; apply schema types when reading pypsa-friendly tables #138) and the fix belongs at the read boundary; an xfail test records it here.
• I also realised that I was using timeslice and timeslice_id as column names, this PR works around it, but Rename the timeslices table's timeslice_id column to timeslice #140 will clean up to just use timeslice everywhere.

src/ispypsa/pypsa_build/links.py                          → new optional args + the per-snapshot expansion
src/ispypsa/translator/network.py                         → zero-capacity corridors get per-unit 0 instead of being skipped
tests/test_model/test_add_links_with_timeslice_limits.py  → new
tests/test_translator/test_network.py                     → zero-capacity test now expects the 0 rows, in the templater's named-timeslice shape

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/ispypsa/pypsa_build/links.py	100.00% <100.00%> (ø)
src/ispypsa/translator/network.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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