monitor memory and do gc

[petrelharp]

See https://tskit-dev.slack.com/archives/CVD28GG5T/p1638499698160200 and surrounding discussion - pyslim.load( ) needs some manual gc, seems like.

[benjeffery]

We have some big changes coming down the pipe to do with memory management, hoping those will fix this.

[bhaller]

Obsolete I think? pyslim.load() is not a thing any more, right?