threshold.custom_range outputs and CMYK

[annacasto]

Describe the bug
When we used pcv.threshold.custom_range and only defined one output (I think it's expecting 2), it didn't give an error, but it returns a tuple that you then can't use as a mask downstream. It's obviously not how the function is supposed to be used, but it was not intuitive that there was no error but the output of custom_range wasn't useable as a mask.
custom_range also doesn't currently support CMYK colorspaces.

To Reproduce

1. Read in a grayscale image.
2. Convert to V channel in HSV colorspace.
3. Threshold using threshold.custom_range but only define one output.
4. Printing type(mask) shows "tuple" and there is no error about expecting 2 outputs.

Expected behavior
I think generally pcv functions throw an error if a user mistakenly defines the wrong number of outputs for a function.

Local environment (please complete the following information):

• OS: macOS
• Environment: conda
• PlantCV Version 4.3.2

Additional context
Add any other context about the problem here.