`load_document` conveniance functions

[mr-c]

Python has

• load_document() taking a URL or a YAML object
• load_document_by_string() parses the provided string into YAML and then into a CWL object using the provided base URI
• load_document_by_yaml takes only a YAML object and a base URI

Java has

• loadDocument() supporting loading from a Map, java.nio.file.Path, java.io.File, String (unparsed YAML/JSON)

All functions will parse any of the schema objects marked documentRoot: true, which are provided to the epilogue function of the cpp codegen (but is sadly, currently ignored).

With load_document() implemented I can work on #5 "Test CWL loading with valid documents from the conformance tests"