Extract convert_unit function out of lammps_engine.py into MD engine common module

**Is your feature request related to a problem? Please describe.**
The convert_unit function in the lammps_engine.py module converts between the MDMC system of units and the LAMMPS system of units. This function is sufficiently general that it could be applied to all MD engine facades.

**Describe the solution you'd like**
Create an MD engine common module for functions and classes common to all MD engines. Extract the convert_unit function from lammps_engine.py and include it in this common module. Add a system of units dictionary to mmtk.py and implement unit conversion with unit_convert. Follow the same approach for all future MD engines.