Parallelize refinement check, quotienting and pruning

[ulriknyman]

Ensure that all the underlying data structures are thread safe and can be used in a multi-threaded context.
Make use of multi-threading in the refinement check, quotienting and pruning.

[seblund]

Fortunately, all safe rust code is thread safe, so we will not have to do any major changes to data structures once #111 is done.

Without #111 all DBM operations are behind a global mutex because UDBM is not thread safe, so we have little to gain from multi-threading in these areas until then.