add back reduceChords early version

reduceChordsOld — needed to bring great reduceChords speed up to
performance capability of older system.
MEI: Style cleanup (just minor things)
MEI: Add durations to beamFromElement() doctest

This doctest breaks if the default duration is changed, so we'll just be sure to
set it.
MEI: Mark no-duration <mRest> and <mSpace> with "m21wasMRest"

When an <mRest> or <mSpace> element does not have a @dur attribute, its value
cannot be correctly determined at creation time. By adding the "m21wasMRest"
attribute to the Rest or SpacerRest returned by mRestFromElement(), we can find
and correct these objects.
MEI: New strategy for <mRest> and <mSpace> without @dur

With a little cooperation between MeiToM21Converter.run(), measureFromElement(),
and a brand new function (that's even tested!), I believe I've solved the
problem of <mRest> and <mSpace> elements that don't have a @dur attribute!
Merge branch 'master' into mei