Catch wider relative URI's when converting RO-Crate to graph object

[douglowe]

This also needs to catch ids like data.csv (relative URIs which may not have ./ or / at the beginning)

Also there should not be ids beginning with ../ - this is forbidden in RO-Crate

Originally posted by @elichad in #23 (comment)

[douglowe]

Blank node: e.g. _:alice
Relative path: e.g. outputs/data.csv or data.csv or either of those with ./ or / at the start...
Local identifiers: #alice or alice