can't use locally defined callables/objects with multithreaded

[LoannPeurey]

Is your feature request related to a problem? Please describe.
bugfix, using a local function for import or other similar features in a multi threaded environment results in failure (can't pickle local objects). The issue seems to be related to multiprocessing's way of handling things. Test coverage isn't catching this

Describe the solution you'd like

• Explore alternative that allow this (multiprocess?)
• extend test coverage to catch this issue

[LoannPeurey]

Putting this into super low priority,
People who want to use the package with custom function can work around this by defining their functions globally. this would require re-factoring quite a lot for a very specific and rare case.