Improve documentation around arrays as values of properties

[Pfeil]

This issue is opened as a response to #215.

• document the purpose of add* and set* methods in builders and their difference
• include array tests like encodingFormat if not already available and test/demonstrate if we can do this with the current API:

{
    "@id": "1st-tool.cwl",
    "@type": "File",
    "name": "First executable tool",
    "description": "An example Common Workflow Language File",
    "contentSize": "120",
    "encodingFormat": ["text/plain", {"@id": "https://www.commonwl.org/v1.0/Workflow.html"}]
  }

• There was no code given which generates this, but make sure this error message is only given in appropriate contexts:

[ {
  "@id" : "xsd:string"
} ]
does not comply with the flattened structure of the RO-Crate json document.