investigation: requirements for removing mmtk dependency

[franzlang]

The mmtk module that MDANSE depends upon is only available for python 2 (e.g. see the documentation here:http://dirac.cnrs-orleans.fr/MMTK.html). investigate and document what is required to remove this dependency and create separate issues for any parts of the MDANSE code that need to be updated/changed.
A separate issue (#6) exists to investigate what would be required to move from a custom version of mmtk provided by the ILL to the standard version of the modules; such a move might be deemed pointless depending of the outcome of this investigation.

[RastislavTuranyi]

MMTK.Units is a collection of constants and should run without any issues with python 3. Therefore, the file can be copied to Externals/ as-is, or the relevant constants can be defined in the MDANSE file that needs them. Either way, the following file will have to be edited:

Classes that use MMTK.Units

MDANSE.Framework.Jobs.Castep
MDANSE.Framework.Jobs.DCDConverter
MDANSE.Framework.Jobs.Density
MDANSE.Framework.Jobs.DL_POLY
MDANSE.Framework.Jobs.Discover
MDANSE.Framework.Jobs.Forcite
MDANSE.Framework.Jobs.Gromacs
MDANSE.Framework.Jobs.LAMMPS
MDANSE.Framework.Jobs.MaterialsStudio
MDANSE.Framework.Jobs.McStasVirtualInstrument
MDANSE.Framework.Jobs.Temperature
MDANSE.Framework.Jobs.VASP

[RastislavTuranyi]

The main problem with removing MMTK entirely is how to replace the in-memory representations of MD data. This is the primary usage of MMTK in MDANSE and coding it from scratch is essentially writing a new package in addition to altering all the affected MDANSE code (as documented in issue #6 ). Indeed, there exist multiple packages that serve this role. The replacement of MMTK with some of these has been investigated in issues #13 (MDAnalysis) and #14 (Mosaic). Using these seems like a much more reasonable way of removing MMTK than writing its functionality anew.