Ap-features

What's Changed
* Delete C - code
* Rename arguments in `apd_up_xy` from `factor_x` and `factor_y` to `low` and `high` respectively
* All functions in the `features` module that takes a factor now prefer the value as an integer. E.g you should rather to `decay_time(a=80)` rather than `decay_time(a=0.8)`, even though the latter also works.

What's Changed

* Remove C-extension and add type signatures to numba functions finsberg in https://github.com/ComputationalPhysiology/ap_features/pull/77
* Update README
* Move some test functions into new `testing` module.


**Full Changelog**: https://github.com/ComputationalPhysiology/ap_features/compare/v2023.1.3...v2023.2.0