Negating Arbitrary Text Operators

[tarungog]

It's currently not possible to negate arbitrary text operators like 'contains_word', even though the API supports this. library only lets you use not_equal

[williamgilpin]

Hi Tarun, my apologies for the delay on this---I have not had much bandwidth to resolve these issues that have been popping up with the new RCSB API.

If you happen to know how to implement this (even approximately), and would be willing to post your workaround here (or do a full PR, depending on your preference), I can incorporate it. Thanks very much.