[ENH] Corpus refactoring

[PrimozGodec]

Issue

Originally I started to solve an issue that Corpus.from_file didn't load pickles together with preprocessing, since Corpus was transformed to Table after loading and then back.

It led to the decision to refactor Corpus to use Table's __new__ class method and to match constructors signatures to Tables. There was no other nice way to fix the first error.

Description of changes

Corpus constructor refactoring. All old ways of initializing corpus should still work (the user is warned that it will change in future).

Edit:
After a fix in Orange biolab/orange3#5812 it would even not be necessary to refactor the Corpus, but it is done already (since before this change there was no other way). I suggest that we refactor it anyway since it is implemented already.

This PR needs Orange to be released (it needs biolab/orange3#5812). Tests will not pass before version 3.32 or 3.31.2 is available.

Includes

• Code changes
• Tests
• Documentation

[ajdapretnar]

This doesn't solve the issue of missing preprocessing for me. I tried deerwester, used default preprocessing and saved it as a pickle. Upon loading it with Corpus, the preprocessing is missing.

[PrimozGodec]

This doesn't solve the issue of missing preprocessing for me. I tried deerwester, used default preprocessing and saved it as a pickle. Upon loading it with Corpus, the preprocessing is missing.

It should be fixed now