get_document_text() and get_document_language()

[reckart]

Is your feature request related to a problem? Please describe.
Coming from Java, I would expect there to be Cas.get/set_document_text() and Cas.get/set_document_language() methods, but there is only sofa_string()

Describe the solution you'd like
Maybe these methods could be introduced and call sofa_string internally?

Describe alternatives you've considered
Just get used to new names ;)