MEI-to-music21 Conversion Is Complete

Okay---it's not "complete" because MEI is a huge specification, but we feel comfortable asking to merge it into music21 proper.

We are still working on adding tests for the [sectionScoreCore()](https://github.com/ELVIS-Project/music21/blob/bbe72a5e08664e085a02b491bbff4260999cef85/music21/mei/base.py#L2755) function, feeling there are currently too few. Aside from this, we expect all future changes will be submitted as their own feature-specific pull requests, since they are unlikely to change large portions of the code.

We believe we have incorporated your feedback from September. Please let us know of any additional changes requested before you accept the pull request, and we will try to accomodate them as much as possible.

Thank you!