Scale pixels to SI units

[sfmig]

Is your feature request related to a problem? Please describe.
We would like to make it possible for users to express the arrays expressed in pixels in a dataset (or per array?) to SI units.

Additional context
Somewhat linked to this issue on unit-aware xarrays

[niksirbi]

One way to achieve this in a more general way would be via the envisioned transforms module.

An example follows:

from movement.transforms import scale

# Convert pixels to cm (assume 10 pix/cm)
# Passing a float/int as a factor results in uniform scaling across all `space` dims
ds.position = scale(ds.position, factor=0.1, space_unit="cm")  # store space_unit in position.attributes

The advantage of this approach is that it would also allow arbitrary scaling factors, not necessarily to metric units.

# Normalise coordinates to a 0-1 range covering a ROI named "arena"
arena = {"xmin": 10, "xmax": 690, "ymin": 20, "ymax": 580}
arena["xsize"] = arena["xmax"] - arena["xmin"]
arena["ysize"] = arena["ymax"] - arena["ymin"]
 # move origin to arena corner
ds.position = translate(ds.position, to=[arena.xmin, arena.ymin])
# scale so that arena extents are in [0, 1] range
ds.position = scale(ds.position, factor=[1/xsize, 1/ysize])

If factor is a scalar, we could interpret it as uniform scale (all 2 or 3 spatial dimensions)
If factor is a list/array-like we could interpret it as 1 factor per x, y, (z) dimensions (length of factor should match number of spatial dimensions).