Allow `pathlib.Path` objects to be used as `root`

[AdamOrmondroyd]

Is your feature request related to a problem? Please describe.
pathlib.Path provides a handy way of dealing with / vs \ on unix/dos-based OSs, but currently anesthetic cannot accept one as a root to read_chains. I also like how / is the append operator :)

Describe the solution you'd like
I would like to be able to use Paths in this way, a well-placed cast to str should do the trick.