Guitar harmonics base class

[laturetab]

Base class (PitchAdjuster) to manage stringed instrument harmonics. Lives within a MuseScore Note.
Follows MusicXML design of base, touching and sounding pitch so it should work for music notation
and/or tablature. Loads/saves to MuseScore file format (mscx) -- updates Ms::Note's read/write methods.
Updates a Ms::Note's ppitch() method to change pitch based on harmonic math. Supports fractional fret
numbers. Supports fixed pitch adjustments as well for future support of TablEdit pitch bends.

This is a base class only -- no UI and no hooks in import code (guitarpro, musicxml). If this class gets
accepted in some form I will update the importers to use it.

• [x ] I signed CLA
• [ x] I made sure the code in the PR follows the coding rules
• [ x] I made sure the code compiles on my machine
• [ x] I made sure there are no unnecessary changes in the code
• [ x] I made sure the title of the PR reflects the core meaning of the issue you are solving
• [ x] I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• [ x] I made sure the commit message title starts with "fix #424242:" if there is a related issue
• I created the test (mtest, vtest, script test) to verify the changes I made

[cbjeukendrup]

Hi @laturetab!
At a first glance, your PR looks good!

One tip for the future: amending existing pull requests is strongly preferred over closing ones and creating new ones. If you close a PR, it is still in the "history" list of pull requests, which makes the list less clear and makes it harder for everybody to find back an old PR.

So if you want to make changes to a pull request, please use git commit --amend and git push -f.

Don't worry about the code style check failing; just make the changes it suggests and amend the PR. When you force-push, the check will be run again, and then it will succeed (if you made all necessary changes).

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