Updated and corrected the wf3cte.rst and wf3cte.py files

[mdlpstsci]

Significant updates to the wf3cte.rst file.

Modified the wf3cte.py file as follows:

• Removed background documentation on the CTE algorithm which does not belong in this source file. Replaced with URLs for handy reference, particularly so the documentation is not duplicated in multiple locations, and the latest information is available for viewing.
• Removed support for incorrect items from the source: ability to set the output filename and ability for the code to accept an association table as input.

Removal of the output filename modifies the API.

[mdlpstsci]

@cshanahan1 @FDauphin I am starting to prep for a new build release candidate which can include as many WFC3 changes that can fit. This PR needs a re-review. Thanks.