Fix #1561: Let makeTies iterate over parts

[TimFelixBeyer]

Fixes #1561

Allows makeTies to iterate over substreams and includes a regression test.
Let me know if there is anything you'd like to see addressed!

[coveralls]

coverage: 93.03% (+0.003%) from 93.027% when pulling e9c87a5 on TimFelixBeyer:patch-5 into 9951a8e on cuthbertLab:master.

[mscuthbert]

I think it would be better if makeTies were overloaded in Score and Opus -- I prefer superclasses not to need to think about subclass behavior (as much as possible -- there are already exceptions in the codebase, but I'm trying to remove them rather than add more). It may be that Score, Part, Measure, etc. someday are moved to a different file from stream.Stream.

[mscuthbert]

Hello -- just as a note (from the music21list Google Group) I'm taking a sabbatical from reviewing music21 issues and PRs until at least Jan 1, 2024, so this PR will be deferred until then.

[mscuthbert]

Turns out that it's not possible to overload this easily within Score/Opus, so it's fine as is for those parts.

However, the newly added if returnObj.hasVoices() part was untested and the code below already took into account streams with voices. I'm removing it for now. The Coveralls test results will show you which lines of code have been added or edited and are not yet tested. I added tests for Score/Opus, but until Voices is tested cannot add that.

I'm merging this once all tests pass and then will consider requests again after sabbatical.