Test DT* objects being drop-in replacements of DocTest analogs

[ev-br]

Follow-ups to gh-37

• do not store parts of config stuff, only store it itself (looking at you, DTChecker)
• test swapping DT* things with DocTest* analogs
• document the internals
• finish up testfile
• refactor the find_doctests function, get_public_objects etc into a DTFinder class.

[ev-br]

An actionable item here is to refactor the find_doctests function, get_public_objects etc into a DTFinder class (edited into the top-level description).