issues with extracting bounded sections of cubes (decouple _fix_partial_datetime())

Proposals for a method to extract data from 2 specified points along the time bounds. This could potentially be quite complicated as it would mean de-coupling the lambda construction.

This will be done eventually but it would also require a similar lambda construction for the bounds function. A solution like the one below was originally proposed.

``def extract_bounds(cube, lower_bound, upper_bound):
constraint = iris.Constraint(
time=lambda cell: lower_bound >= cell.point <= upper_bound)
return extract(cube, constraint)``

Start by locating the existing extract and _fix_partial_datetime() entry points referenced in the issue, then compare their lambda construction with the proposed extract_bounds(cube, lower_bound, upper_bound) example. Clarify how bounded cube extraction and bounds handling should be decoupled; done means a decided approach that supports both specified time points and corresponding bounds behavior.

Written by the indexing model from the issue text.