Purpose and Motivation

It's kind of a no-brainer to be able to ask a TempoClock if it's still running -- it might have been stopped, but in the main class library as it is now, you have no way to know, except to try to schedule something on it and catch an error (!).

So I propose a very simple isRunning method. Actually, I've had this in my extensions for years. It works.

Plus documentation.

Plus a unit test... but then I thought, there are some other things about TempoClock (correct rate of advancing through time, handling of meter changes) that are probably not tested anywhere else. So I added a bunch of other useful tests.

Types of changes

• Documentation (non-code change which corrects or adds documentation for existing features)
• New feature (non-breaking change which adds functionality)

Checklist

• All previous tests are passing (well, I did try to run the entire test suite, but TestUGen_Duty locked up the system and forced me to hard reboot... so to be completely honest, I don't know, but I don't think that's the fault of this PR)
• Tests were updated or created to address changes in this PR, and tests are passing
• Updated documentation, if necessary
• This PR is ready for review