dlpoly: check units

go through the [dlpoly_engine](https://github.com/MDMCproject/MDMCv0.2_pilot/blob/master/MDMC/MD/engine_facades/dlpoly_engine.py) script to check that all the units are correct. this includes:
- the unit `System` is correct: https://github.com/MDMCproject/MDMCv0.2_pilot/blob/735bd97c6622f1b1d8d0fac752bd608587c3f185/MDMC/MD/engine_facades/dlpoly_engine.py#L1127
- methods are correctly decorated with units, e.g.: https://github.com/MDMCproject/MDMCv0.2_pilot/blob/735bd97c6622f1b1d8d0fac752bd608587c3f185/MDMC/MD/engine_facades/dlpoly_engine.py#L145
- units in docstrings are consistent with those being used
- the units correctly propagate to the dlpoly-py control object and output

Start with MDMC/MD/engine_facades/dlpoly_engine.py, checking the System unit near line 1127 and the unit-decorated methods near line 145. Review the script's docstrings and trace how units propagate to the dlpoly-py control object and output. Done means the units are correct and consistent across these locations.

Written by the indexing model from the issue text.