Solution: Add helpers for querying task registry

[tomekowal]

Closes: #344

This PR adds functions for querying task registry if specific task is running in the cluster or if any tasks are running.

I have a question about doctests. All documented functions with examples in TaskRegistry module have doctests disabled. I followed what is currently there and listed the two new functions in except. Shall I delete doctests for this module or do you want to have them enabled and listed all functions in except?

Thanks for your time reviewing the changes!

[coveralls]

Pull Request Test Coverage Report for Build 780

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 94.925%

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Totals
Change from base Build 776:	0.1%
Covered Lines:	318
Relevant Lines:	335

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💛 - Coveralls

[c-rack]

Regarding doctests, maybe @maennchen could advise.

[maennchen]

@tomekowal Thanks for your PR! I wanted to specifically disable doctests for functions that can't be properly tested because of state / side effects. The default should always be true.