Lazy evaluation of the amber toolkit version

[vamironov]

Getting _toolkit_version involves expensive system call even if this property is not used. This patch saves few seconds of runtime in conda environments.

• Tag issue being addressed
• Add tests
• Update docstrings/documentation, if applicable
• Lint codebase
• Update changelog

[mattwthompson]

Thanks for the patch - I'll have a look soon

(Don't worry about half of the CI failing, some of our tests require an OpenEye license that's not accessible on forks)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.61%. Comparing base (713cfa9) to head (310c6c6).

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