Mei export v2

[manoskary]

MEI export (first simple version)

This method exports mei without full support for all elements as musicxml supports.

• Necessary to have a single part, or use partitura.merge_parts (maybe can this be changed)
• It supports clef, roman numerals (only for visualization), notes, grace notes, global elements (Time signature, key signature, cleff, staff)
• It supports clef changes
• It supports key and time signature changes only on the beginning of a measure
• It does not support beams unless a beam element is present.
• When note ids are available it assigns them otherwise it uses a unique xml:id

[codecov-commenter]

Codecov Report

Attention: 74 lines in your changes are missing coverage. Please review.

Comparison is base (87585a2) 85.23% compared to head (e108dc9) 84.96%. Report is 9 commits behind head on develop.

Files	Patch %	Lines
partitura/io/exportmei.py	70.51%	74 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #347 +/- ## =========================================== - Coverage 85.23% 84.96% -0.27% =========================================== Files 81 82 +1 Lines 13921 14236 +315 =========================================== + Hits 11865 12096 +231 - Misses 2056 2140 +84 ```

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