When group files derive from another group, we have to know how to
load that group and its supergroups. This interface also knows how
to load interfaces.
Load the group called groupName from somewhere. Return null
if no group is found.
Load a group with a specified superGroup. Groups with
region definitions must know their supergroup to find templates
Specify the template lexer to use for parsing templates. If null,
it assumes angle brackets <...>.
Load the interface called interfaceName from somewhere. Return null
if no interface is found.