openego / eDisGo / 33615564738

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fix(tools): do not raise on a district heating area without a heat pump

`aggregate_district_heating_components` picked the components of an area with
more than one power-to-heat unit by indexing `[0]` into a sector filter:

    district_hp = district_hps[district_hps.sector == "district_heating"].index[0]
    district_rh = district_hps[
        district_hps.sector == "district_heating_resistive_heater"
    ].index[0]

Both raise `IndexError: index 0 is out of bounds for axis 0 with size 0` when
the area holds several units but none of that sector.

That is reachable from real data, not just a constructed case. Central heat
pumps and central resistive heaters are fetched by independent per-carrier
queries and integrated independently in io.heat_pump_import, including a
fallback that integrates a resistive heater as a component of its own when it
cannot be attached to an existing heat pump. An area holding two resistive
heaters and no heat pump therefore exists, and the whole generator import of
that grid died on it.

Both filters are now evaluated once, up front. An area that does not hold one
unit of each type keeps its components and gets a warning naming the sectors
it does contain; the heat demand target falls back to the first unit, which is
what the single-unit branch already did. Areas with one heat pump and one
resistive heater are unaffected.

The feed-in subtraction still happens for such an area — only the merge is
skipped, since there is nothing to merge into.

The defect predates this branch. It was unreachable while the function had no
production caller; the `aggregate_district_heating` task added earlier on this
branch puts it in the path of every district heating run.

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9496 of 11331 relevant lines covered (83.81%)

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