Add MusicXML sound tag parsing (fix #1578)

[TimFelixBeyer]

This PR adds functionality to parse <sound tempo='x'> tags from MusicXML files into MetronomeMark objects (fixing #1578).
bach/bwv66.6 contains such a <sound tempo='x'> tag and is heavily used in doctests. Therefore a lot of tests had to be updated, leading to quite a lot of changes, however nothing seems to be broken.
The only tests that had significantly different outputs are related to feature extraction.
They change because the changed tempo leads to a (very) different beat histogram.
Previously, the tempo was 120bpm, leading to beat bpm values of 60, 120, 240, where 240 was discarded due to the this line:

music21/music21/features/base.py

Line 504 in acf926c

if thisBPM < 40 or thisBPM > 200:

The accurate tempo is 96, so the values are 48, 96, 192 and are therefore no longer discarded.

Let me know what you think!

[coveralls]

Coverage: 93.115% (+0.009%) from 93.105% when pulling 18fdfd6 on TimFelixBeyer:patch-8 into 2f6833a on cuthbertLab:master.

[mscuthbert]

Approved! This is a huge improvement. Thank you! I had no idea there were so many XML files with tempos specified in <sound> elements but without metronomes. I'm going to close and re-open to see if there are any conflicts with the stripTies bug fix.

[mscuthbert]

Indeed - - the typing did catch a minor bug and added a warning that something that appeared typed was not. Thanks for being totally on top of this!

[TimFelixBeyer]

A file in the corpus (schoenberg/opus19) had an incorrect tag, I have removed it.
In addition, I have changed the existing way metronomes were imported to prefer ints over floats where possible to allow for better braille compatibility.

[mscuthbert]

It also just looks more musical to see qtr=144 than 144.0, and
more accurate to the composer's notation. So good!

[mscuthbert]

Sorry for the delay in merging -- I just needed to check out the Schoenberg file and make sure all was good.

[mscuthbert]

I'm changing this a little bit in the next PR because I'm seeing phantom metronome marks in lots of pieces that should not show them in scores (like medieval/Renaissance music, Bach Chorales, etc.) -- these only set numberSounding so that the metronome mark does not appear in the score, just affecting playback.