parallel_embeddings.py and feasibility.py minor corrections

[jackraymond]

Minor improvements to new parallel_embeddings.py and feasibility.py modules:

• max_num_emb should be optional, allowing a None value that indicates no bound on the number of embeddings. This has been implemented.
• The lattice_size() function is merely a helper function for find_sublattice_embeddings() and doesn't have much generalization power beyond this. For this reason I have moved it into the parallel_embeddings.py module. Separately, a new non-trivial upper-bound function could be added to feasibility.py as a feature enhancement (noted in comments).