Implement Validation Phase Ruleset

[EttoreM]

This PR addresses #44.

Rulses Implemented

#	Rule	sh:severity
1	The property additionalType of the ValidationCheck entity MUST be set to https://w3id.org/shp#ValidationCheck (i)	sh:Violation
2	A validated crate SHOULD include a ValidationCheck entity describing the validation result (ii).	sh:Warning
3	RootDataEntity --> mentions SHOULD reference ValidationCheck	sh:Warning
4	ValidationCheck MUST be of type AssessAction	sh:Violation
5	The name of the ValidationCheck MUST provide a human-readable summary of the review and result (ii)	sh:Violation
6	ValidationCheck --> object SHOULD point to the root of the RO-Crate	sh:Warning
7	ValidationCheck --> instrument SHOULD reference an object with @id https://w3id.org/5s-crate/0.4	sh:Warning
8	ValidationCheck SHOULD have actionStatus property	sh:Warning
9	ValidationCheck --> actionStatus MUST have one of the allowed values	sh:Violation

(i) This is not an actual rule, but rather a definition of what constitutes a ValidationCheck entity. As such it was not implemented given its axiomatic nature.
(ii) This rule is implicitly included in rule 3, hence it was not implemented.
(iii) This cannot be entirely done without a manual check. Here we only check that the name property of ValidationCheck is a string of at least 10 characters.

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Rules Not Implemented (need clarifications)

#	Rule	sh:severity
1	ValidationCheck --> object MAY point to additional entities, as required by TREs/workflows, as an array of objects.	sh:Info
2	ValidationCheck --> instrument MAY reference additional expected input parameters, as required by TREs/workflows, as an array of instruments	sh:Info
3	ValidationCheck -->object MAY reference additional checks of provided input parameters against expected, as required by TREs/workflows, as an array of objects	sh:Info

[douglowe]

We will leave the unimplemented rules to be covered by a future PR.

[douglowe]

Looks pretty good - there's a couple of changes I've noted in comments.

I also think we should be including tests for startTime and endTime - as mentioned in the main text of the 'Review Process' section. We perhaps can add these later though.