Parameter file expects `BoxSize` even if `PeriodicBoundaryOn 0`

[VictorForouhar]

Found when developing the bonsai branch, which is a non-cosmological and non-periodic simulation.

HBT-HERONS compiled using git branch: bonsai_virial_units and commit: 7acb7429544f2d8fcd490df29eede0674c4f5922 (with uncommitted changes)
Reading configuration file ../examples/configs/bonsai/bonsai.conf
terminate called after throwing an instance of 'std::invalid_argument'
  what():  BoxSize: Mandatory parameter not found.

We should not make BoxSize mandatory if we do not use periodic boundary conditions. We should also not make it mandatory if we read it directly from the snapshot (as is the case for certain simulation formats, e.g. swift).