Tesseroid forward modeling takes an optional open multiprocessing.Pool

[leouieda]

The forward modeling functions in gravmag.tesseroid take an option
multiprocessing.Pool object to run things in parallel. This way, a pool
object can be reused for multiple calls, avoiding the repeated spawning
of processes.

Refactored the function code in the process to have less repetition. The
_dispatcher function now: checks arguments, spawns the pool if
necessary, dispatches the work, and returns the result.

Fixes #182

Checklist:

• Make tests for new code
• Create/update docstrings
• Include relevant equations and citations in docstrings
• Code follows PEP8 style conventions
• Code and docs have been spellchecked
• Include new dependencies in docs, requirements.txt, README, and .travis.yml
• Documentation builds properly
• All tests pass
• Can be merged
• Changelog entry