typo in the check_swapping_css_inputs() function

[jrich129]

Lines 555-556 in subset_selection.py state:

    if len(exclude) != len(set_include):
        raise ValueError("Exclude has repeated elements.")

The actual if statement should read:

    if len(exclude) != len(set_exclude):
        raise ValueError("Exclude has repeated elements.")

This pattern may appear in more than one place in the same file (search for len(exclude) != len(set_include)).