ipcc_likelihood_scale - clarify left_open and right_open arguments

[efcaguab]

I find left_open and right_open slightly confusing.
Maybe either rename these (e.g. left_includes/right_includes or left_excludes/right_excludes). I would prefer left_includes/right_includes to avoid double-negatives where it is FALSE.
An alternative option is to add some further info to help (e.g. left_open = "whether the left limit is an open interval, i.e. excludes the left bound)