Allowing model.id_map to return overlap ID values

[shimwell]

This small PR adds the option to use the color_overlaps argument when calling the id_map function. It looks like this argument is well supported upto the openmc.lib layer and then not in the final user facing id_map function. It would be useful for me to find overlaps using this function.

I also added a simple test to check -3 (which is the overlap ID number) appears in the returned numpy array only when the flag is used. I set the default to fault to preserve current behaviour

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)