21 referencing a workflow crate

[EttoreM]

Rules implemented

Description	sh:severity
The RootDataEntity MUST have the schema:mainEntity property with cardinality 1.	sh:Violation
The mainEntity pointed to by the RootDataEntity MUST be of type schema:Dataset.	sh:Violation
The mainEntity MUST have a purl:conformsTo property with an IRI starting with "https://w3id.org/workflowhub/workflow-ro-crate"	sh:Violation
If mainEntity has an HTTP(S) @id, it SHOULD have a distribution that is an HTTP(S) URL.	sh:Warning

---

Rules NOT implemented

There are other rules pertaining to the mainEntity, but these were deemed to be not implementable.
For sake of completeness these are reported below

Description	sh:severity
The @id of the mainEntity SHOULD be a permalink or versioned URL	sh:Warning
The @id of the mainEntity MAY be a nested directory in the payload.	sh:Info
Workflow programming language and detailed metadata MUST be provided in the referenced Workflow RO-Crate, not this crate	sh:Violation
If present in the referenced Workflow RO-Crate, SHOULD be either CWL or Nextflow URIs.	sh:Warning
If the identifier is not a URI, Clients SHOULD follow Signposting headers to find the rel="item" link to the RO-Crate ZIP file	sh:Warning

[EttoreM]

@elichad @douglowe @stain @alexhambley

I think this PR is ready for review.

[alexhambley]

Should be single colon :

[EttoreM]

Done

[alexhambley]

I'm not 100% sure, but I think this may only validate the prefix when conformsTo exists. In other words if conformsTo is missing, the violation might not report (?)
sh:minCount 1 ; might be required. Worth a test to see.

[EttoreM]

You're right. Added a constraint for that and a corresponding test.

[alexhambley]

Provided comments in-line. There is a double colon typo and conformsTo property needs checking :)

[douglowe]

This is superceded by PR #28