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

[douglowe]

We need to catch all relative paths and make sure that they remain relative when they are loaded into the graph. This means reversing the absolute paths that may have been created during RDFLib parsing back to relative.

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