add support for saving computational resources used to .trees files

[bhaller]

See the proposed schema change here: tskit-dev/tskit#3016. SLiM should save out this info, if the PR is approved.

[hyanwong]

I was looking to try and extract the system (or total) time take for a SLiM simulation, from the output tree sequence. I was thinking this could be done simply by storing the simulation start time in the SLiM metadata, but maybe storing the provenance data in the format in tskit-dev/tskit#3016 would negate the need for that.

The PR seems to have been approved, just waiting to merge in, and next week we should have the new tskit version (0.5.9) with this included.