Add load() API

[JPEWdev]

Adds an API to automatically load an SPDX document with the correct model based on the document context. This allows users to load a document without needing to know what specific version it is.

[bact]

May optionally add another test data file with @context that will be surely not in the generated _CONTEXT_TABLE.

tests/data/bad-context/no-context.spdx3.json:

{
  "@graph": []
}

tests/data/bad-context/wrongtype-context.spdx3.json:

{
  "@context" : 1,
  "@graph" : [ ]
}

tests/data/bad-context/unknown-context.spdx3.json:

{
  "@context" : "https://example.com/context.jsonld",
  "@graph" : [ ]
}

All of the three should raise LoadError