Nrefocus

- fix: `autofocus` method of Refocus was not functional
- ref: use `Refocus.autofocus` for legacy autofocus method
- docs: fix rtd builds

- feat: implement nrefocus.RefocusPyFFTW for faster refocusing
using pyfftw (11)
- enh: speed-up propagation kernel computation using numexpr
- docs: cleanup

- dist: include submodules in wheel/dist

- feat: introduce nrefocus.RefocusNumpy and nrefocus.RefocusNumpy1D
interface class for user-convenience and efficiency
- docs: cleanup
- ref: new submodule for metrics and metrics now accept a Refocus
instance as an argument
- ref: new submodule for minimizers and minimizers now accept
a Refocus instance
- ref: make legacy autofocusing code use the new Refocus class

- fix: fix several minor bugs (deprecations?) that caused the
tests to faile
- ci: migrate to GitHub Actions
- setup: setup.py test is deprecated
- docs: refurbish documentation

- Drop support for Python 2 (8)
- Code cleanup