Parallel Design Doc

[trisyoungs]

This PR adds a preliminary design document covering parallelism as-implemented within Dissolve, and requirements going forward.

[rprospero]

I think we need a bit more on what we expect from the parallelism.

[DanNixon]

For me the big thing missing here (which @rprospero has already mentioned to some extent) is a description of the problem you are actually trying to solve with parallelism.
As such there isn't enough detail here to determine what a good solution would be.

E.g.: you mention some constraints around shared data but do not mention quantity, if this data is mutated, etc.