subset indexing deterministic across openMP threads. no early termination of multiwindow neighbour list generation.

[JoelPhys]

Description

• updated some var names in dicmultiwindow_util.cpp to be consistent with other parts of DIC code.
• Make subset indexing deterministic across OpenMP threads.
• prevented neighbour list generation exiting early when a subset was found that had 0 neighbours.

Type of change

• [ X] Bug fix (non-breaking change which fixes an issue)
• [ X] New feature (non-breaking change which adds functionality)