WIP-MEI: Formatting: Rewrite implemented/testing/unimplmemeted for elements

This is more concise, using fewer lines to describe what hasn't yet happened.
Further, the section headings will now be rendered in a readable way by
Sphinx when the API is built.
WIP-MEI: Refactored slur and tie pre-processing for speed

This strategy is much faster, since the majority of the element tree is only
iterated once, rather than twice as many times as the number of slurs and ties.
WIP-MEI: Formatting: Minor cleanup thanks to pylint
WIP-MEI: Tests: Added tests for @grace in noteFromElement()

These tests can be expanded later to test other attributes too.