In transition interface sampling and related methods, we need to define a
set of interfaces for each sampling transition. In OPS, this is essentially
just a list of volumes. However, it can be useful for that list to carry
some addition information (such as the associated collective variable), so
InterfaceSet is a way to package that information.
List of volumes representing a set of interfaces, plus metadata.
Abstract class for InterfaceSets for CVRange-based volumes.