Add in_mm3 in remove_small_objects

[KumoLiu]

Fixes #7034.

Description

Add in_mm3 in remove_small_objects, if in_mm3=True, min_size will be divided by pixdim when the image is MetaTensor.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[KumoLiu]

In this PR, I added the physical_scale option.

• If "physical_scale=True," "min_size" will be represented by physical scale, which means, if "min_size" is 2, the min-size will be divided by pixdim to represent the number of pixels in 2mm^3. Then this transform will remove the objects smaller than 2mm^3.

@AHarouni, could you please check whether this one can help address your issue?

cc @wyli @ericspod

[wyli]

Thanks, it looks good to me.

[wyli]

/build

[ericspod]

This assumes the images are 3D, what about 2D? mm^3 isn't the unit being used in that case. Also we haven't specified what units we're working in, I would just state that the measure is in unit area or volume and not assume units are mm. Typically we resample in terms of square or cubic mm but not always and we don't encode in types or anywhere else what units we're dealing with, they've elsewhere always been implicit.

[wyli]

/build

[wyli]

/build