`restore_simulation` silently drops memberless group entities

[Adelagric]

policyengine_core/tools/simulation_dumper.py (inherited verbatim from openfisca-core) rebuilds entity counts from the members mapping instead of the dumped ids:

person_count = len(population.members_entity_id)
population.count = max(population.members_entity_id) + 1

Consequences, all verified against the equivalent code in openfisca-core:

1. A group entity with no member is silently dropped from the restored simulation's count, while still listed in its ids — an internally inconsistent state. Calculations on the restored simulation then return arrays with one entity missing, with no error. If a value was stored for the group entity, restoring instead fails with ValueError: length is N while there are N-1 ….
2. A tax-benefit system with no group entity cannot be restored at all: person_count is never assigned and restore_simulation raises UnboundLocalError.

Upstream report and reference fix (restore each entity's count from its own dumped id.npy, which also removes the person_count plumbing): openfisca/openfisca-core#1387.

Happy to open the twin PR here if useful.