CRAN v0.1.0: Enhance RO-Crate handling and validation functions

[villegar]

Overview:

• Added higher level function to load RO-Crates from various inputs, load_rocrate.
• Added higher level function to validate RO-Crate and display report with errors and warnings (if any was found), validate_rocrate.
• Defined new checks for RO-Crate validation to ignore system specific directories (e.g., __MACOSX on macOS).
• Refactored functions to streamline checks, including is_rocrate.
• Updated get_entity so it takes a vector for id and type, reuses the shorter vector (only if the length 1), or errors if vectors have different lengths.
• Updated is_rocrate_bag so it returns a boolean value, indicating the validity of the given RO-Crate bag. Previously, it returned a path to the root of the RO-Crate.
• Updated unbag_rocrate to return the path to RO-Crate root visibly.
• add_entities() is deprecated. Use add_entity() instead.
• load_rocrate_bag() is deprecated. Use load_rocrate() instead. Note that validation must be done as a separate step using validate_rocrate().
• read_rocrate() is deprecated. Use load_rocrate() instead.
• remove_entities() is deprecated. Use remove_entity() instead.
• entity(x=) renamed to entity(id=).

Closes #5 and closes #7

[villegar]

@elichad just to let you know that this is a new release of the package, now at a more mature stage! Which has also been released on CRAN.

[elichad]

@villegar thanks for letting me know :) I can't contribute much in the way of helpful review here - so my main question is just whether all the new features have been documented. I guess that they have been, as I see updates to the docs and a new vignette - so I have no objection to merging