Implemented rules for the Workflow Execution Phase.

[EttoreM]

This PR addresses #37

Rules implemented

#	Description	sh:Severity
1	RootDataEntity SHOULD mention a Workflow Execution object (i)	sh:Warning
2	The Workflow Execution object MUST provide a human-readable name (ii)	sh:Violation
3	Workflow Execution object MUST be of type CreateAction (iii)	sh:Violation
4	The Workflow Execution object MAY have the startTime property if it has begun.	sh:Info
5	The startTime of the workflow execution object MUST follow the RFC 3339 standard.	sh:Violation
5	The Workflow Execution object SHOULD have the endTime property if it has ended.	sh:Warning
6	The endTime of the workflow execution object MUST follow the RFC 3339 standard.	sh:Violation
7	The endTime of the workflow execution object MUST follow the RFC 3339 standard.	sh:Violation
8	The Workflow Execution object MUST have an actionStatus that reflects the current execution state.	sh:Violation

(i) The Workflow Execution object is the only entity in this phase that is typed schema:CreateAction.
(ii) This was implemented by requiring that the name of the Workflow Execution object is of type string and is at least 10 characters long.
(iii) This rule is implemented implicitly within the others. For example the RootDataEntity MUST mention a CreateAction entity which is assumed to be the Workflow Execution object in this phase (there is only one CreateAction object in the crate's graph).

[elichad]

I found some issues with the tests - but the SHACL looks good to me, so hopefully there are no major issues hiding :)

Aside from that, just a few small tweaks suggested.

[elichad]

One more suggestion - just a simplification to make future maintenance easier.

Otherwise, this looks good to me, so approving.