Changed the method name of the betweenness function from "minimum" to "infimum".
This reflects the fact, that this variant of betweenness actually computes the infimum (i.e., the limit) rather than the minimum (i.e., the actual value) over all alpha-cuts, because the latter may not exist. No changes have been made to the code's logic, this is merely a more consistent naming. The old value "minimum" can still be used (i.e., the code is backwards compatible).