Log REST2 scale factors and selection atoms

[lohedges]

This PR adds selective logging for the REST2 scale factors and selection. These are only logged when there are any non-zero REST2 scale factors. If the selection is None, then the entirety of the perturbable region is used as the selection. This is handled within the Sire layer, so we need to catch this here and log the atom indices too.

Running a command such as:

somd2 system.bss --rest2-scale 2.0 --rest2-selection "molidx 0 and (not atomidx 0,1,2)"

Will give output like:

2025-12-11 14:18:59.821 | INFO     | somd2.runner._base:__init__:122 - somd2 version: 2025.2.0.dev
2025-12-11 14:18:59.822 | INFO     | somd2.runner._base:__init__:123 - sire version: 2025.4.0.dev+ac2408f
2025-12-11 14:18:59.873 | INFO     | somd2.runner._base:__init__:267 - Applying modifications to ghost atom bonded terms
2025-12-11 14:18:59.925 | INFO     | somd2.runner._base:_check_end_state_constraints:797 - Constraints are at not the same at λ = 0 and λ = 1.
2025-12-11 14:18:59.932 | INFO     | somd2.runner._base:__init__:406 - REST2 scaling factors: [1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 1.8, 1.6, 1.4, 1.2, 1.0]
2025-12-11 14:18:59.934 | INFO     | somd2.runner._base:__init__:431 - REST2 selection contains 5 atoms: [3, 4, 5, 6, 7]

@mark-mackey-cresset