Add helper for printing Umpire pool statistics

[msimberg]

This is the non-production ready debugging version that I used to print Umpire pool statistics for debugging. It needs cleanup to be ready for merging, but more importantly if we want something like this in master we should:

• decide how to trigger the printing and when (when finalizing DLA-Future, after every algorithm, with or without synchronization, environment variable)
• decide which statistics to show and in what form

[albestro]

Thanks! We might think of taking chance of this also for tracking memory usage in our benchmarks (which should just add to requirements "easy to be parsed").

[msimberg]

Thanks! We might think of taking chance of this also for tracking memory usage in our benchmarks (which should just add to requirements "easy to be parsed").

I think this is a very good idea. I could see at least three different directions:

• trigger printing of statistics at the end of each algorithm
• explicitly call a generic function to print statistics at the end of all miniapps
• above, but implement a specialized format only for miniapps that's easier to parse

[msimberg]

Closing just to tidy up my open PRs. This can always be revived later. I'm not deleting the branch.