Add type hints and support for arithmetic operators + and - on ValueTracker

[fmuenkel]

Overview: What does this pull request change?

• Add type hints to valuetracker.py
• Add error handling for __iadd__() and __isub__()
• Add __sub__() to solve mypy error for __isub__()
• Small improvements to docstrings

Motivation and Explanation: Why and how do your changes improve the library?

Related to #3375

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[chopan050]

LGTM! Thanks for these changes!

[chopan050]

Sorry - I did not notice the __sub__ method when I approved the PR earlier. I understand that MyPy requires it. However, there is something I have to say about the implementation:

[chopan050]

LGTM! Thanks for the changes!

Now that __sub__ and __add__ is implemented, I noticed that we should do the same for __mul__, __truediv__, __floordiv__, __pow__ and __mod__, since their inplace counterparts are implemented. Now, since this is mainly a PR for adding type hints, this should be done in a subsequent PR. Would you like to do that PR as well?

Meanwhile, I'll just approve and merge this PR.

[fmuenkel]

Sure, I will have a look at it..