[spossann]

Fixes #852.

Problem

A second Simulation.run() on the same object (the restart in the model tests, utils_testing.call_test, or a notebook that runs twice) saved stale scalar quantities: every value equal to the last value of the first run. run() calls allocate() again, which creates new spline vectors, mass operators and particles, but the scalars had cached the objects of the first run on their first update (hasattr checks in _local_update).

Fix

• Scalar.allocate(): new hook that fetches the objects created by the allocation; update() calls it once after each allocation. BilinearEnergyFEEC, VolumeFormEnergyFEEC and PICScalar (clone config) fetch their objects there instead of caching them in _local_update. The Scalar docstring states the rule.
• Scalars.reset_allocation(): marks every scalar, including the summands of sums such as en_tot, for a new fetch; called at the end of Simulation.allocate().
• Model-side caches with the same pattern move to post_allocate: _divUv of VariationalMHD and VariationalCompressibleFluid (only for divdiv_alpha > 0), _f0/_f0_values of LinearVlasovAmpereOneSpecies.
• New unit test test_scalars_follow_a_new_allocation.

No baseline changes: the regression data is taken before the restart, and the first run is unchanged.

Verification

Maxwell, default parameters (Tend = 0.03), electric energy en_E:

	values
first run	[0, 2.159e-5, 8.467e-5, 1.8439e-4]
second run, same object (before)	[1.8439e-4, 1.8439e-4, 1.8439e-4, 1.8439e-4]
second run, same object (after)	[0, 2.159e-5, 8.467e-5, 1.8439e-4]
restart to t = 0.04, last value (after)	3.131146e-4
straight run to t = 0.04, last value	3.131146e-4

Tests run locally:

• src/struphy/models/tests/test_scalars.py (6 passed), test_variational_divdiv.py (5 passed)
• regression tests, serial: Maxwell, VariationalMHD, VariationalCompressibleFluid, LinearVlasovAmpereOneSpecies, LinearVlasovMaxwellOneSpecies, VlasovAmpereOneSpecies, GuidingCenter
• regression tests, 2 ranks: Maxwell, VlasovAmpereOneSpecies, LinearVlasovAmpereOneSpecies

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