add a way for bids.layout to save (and later reload) an indexed dataset layout.

[Remi-Gau]

Similar to what pybids BIDSlayout save and load does.

https://bids-standard.github.io/pybids/generated/bids.layout.BIDSLayout.html#bids-layout-bidslayout

This should help speed things up when reloading large BIDS datasets.

I suspect some people are already doing something like this so might as well implement it here.