WIP-MEI: Tests restored for @grace in noteFromElement()

I accidentally removed the part that tests handling of @grace, apparently for no
reason, in commit ab67d1e325cd06daf0f561fd04952b4cd4d1950c. Now I put it back.
WIP-MEI: Tests added for @grace in chordFromElement()

Basically just copied over the recently-reinstated tests from noteFromElement().
WIP-MEI: Docs adjusted to account for things actually implemented

A lof of the "in testing" things were contained elements. Since I'm not going to
test each contained element individually, I'll just use any tests in general as
sufficient proof that the rest of the contained elements will be taken care of.
Except for functions that don't use _processEmbeddedElements().
WIP-MEI: Added equivalencies for the weird accidentals in _ACCID_ATTR_DICT

I followed the example set by the music21.pitch.accidentalNameToModifier
dictionary, which is used by music21.pitch.Accidental to know which accidental
is which.
WIP-MEI: music21.duration modified for 2048th notes

Since MEI allows 2048th notes, and it's relatively easy to add support in
music21, I've done it. I'm not 100% sure it's perfect.