Improve storage pixel size/scale Python API

[andy-sweet]

In acquire-project/cpx#110, we added a Python API to control pixel size/scale in the zarr storage device. After that PR, you can do something like

p.video[0].storage.settings.pixel_scale_um = (0.5, 4)

to specify the (y, x) pixel sizes in microns.

The main goal of this task is to consider and implement alternatives that may be more ergonomic. For example, one alternative may be to change the name and only allow specification of a scalar:

p.video[0].storage.settings.microns_per_pixel = 0.5

Or we could think more about future support for camera transform parameters and make this consistent with those ideas.