Make MONAI robust to non-default meta dict names

[atbenmurray]

This PR adds a field to the meta dict indicating which tensor it belongs to, and a method get_meta_dict_key to find the meta_dict when it is present in a dictionary.

I have addressed many of the obvious places where there might be issues if the default meta_dict extension is not used but there are likely more that I need to find.

Description

A few sentences describing the changes proposed in this pull request.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.