Unit aggregation should preserve unit ids.

[h-mayorquin]

Like https://github.com/SpikeInterface/spikeinterface/issues/2789 but for UnitAggregation. Currently it does not:

from spikeinterface.core import generate_sorting


sorting1 = generate_sorting(num_units=3)
sorting1 = sorting1.rename_units(new_unit_ids=["unit1", "unit2", "unit3"])

sorting2 = generate_sorting(num_units=3)
sorting2 = sorting2.rename_units(new_unit_ids=["unit4", "unit5", "unit6"])

from spikeinterface.core import aggregate_units

sorting = aggregate_units([sorting1, sorting2])
sorting.get_unit_ids()
array([0, 1, 2, 3, 4, 5])