Nrefocus

- enh: add std and median metrics (19)
- docs: make docs build on Windows

- enh: clarify roi argument for Refocus.autofocus method (17)

- fix: internally normalize focusing distance to the wavelength for
autofocusing with lmfit, because the scale seemed to affect the
minimizer convergence (14)

- BREAKING CHANGE: The "legacy" minimizer is deprecated in favor of
the new lmfit-based minimizer. The default minimizer is now "lmfit".
- BREAKING CHANGE: Removed the "ret_grad" and "ret_d" keyword argument
from autofocusing methods; there is now "ret_grid" and "ret_field"
instead
- BREAKING CHANGE: By default, the minimizers do not anymore return the
refocused field, this can be re-enabled by using the `ret_field`
option
- build: lmfit is now a dependency
- fix: check for None instead of boolean evaluation for
metrics dealing with ROIs

- ref: deprecate minimizer argument
- ref: legacy minimizer now thinks in SI units
- ref: minor cleanup in autofocusing code

- docs: minor improvements