cwlprov:relationship sketch

Together with #1 this attempts to find a way to pre-define domain-specific provenance that would be generated at workflow run time. The idea is define a set of relationships that will be added onto the produced outputs of a step to relate it to other data values or concepts at creation time.

These can use domain-specific ontologies like [EDAM ontology](http://edamontology.org/page) or [BioSchemas](https://bioschemas.org/specifications/), or more generic ones likes [PROV](https://www.w3.org/TR/prov-o/) or [schema.org](https://schema.org/)

``` cwl
#!/usr/bin/env cwl-runner
cwlVersion: v1.0
class: Workflow

inputs:
first_input: File
second_input: long

steps: []

outputs:
first_output:
type: File
outputSource: first_input
cwlprov:relationships:
prov:wasDerivedFrom: [ '#inputs.second_input' ]
prov:wasInfluencedBy: [ '#inputs.second_output' ]

$namespaces:
prov: http://www.w3.org/ns/prov#
cwlprov: https://w3id.org/cwl/prov#

$schemas:
- http://www.w3.org/ns/prov.owl
```