Support for "provided sources" (was input stream as source)

[pmaria]

See discussion at #2 (comment)

The W3C Web Of Things vocabularies that are currently described in RML-IO are aimed at IOT data streams an hypermedia approaches of handling them. They are not a good fit for the purpose of describing a IO bytestream. Furthermore, there are more types of sources that could be provided to a RML processor programmatically, like e.g. an already deserialized JSON node or XML node etc. The options for this can vary across tech stacks and programming languages, so the proposal here is to shift the problem of handling the provided source type to the RML processor. Thus we provide a simple rml:Source type for provided sources:

Proposal:

[] rml:logicalSource [
  rml:source [
    a rml:ProvidedSource ;
    rml:sourceIdentifier "some identifier" ; 
  ] ;
] ;

A RML processor can then expose an API to be able to provide this source for mapping.