the use of bases adapted to complete chain of radical powers slowed down computations

affected methods:
. DefiningIdealOfUpperPartOfUnitaryGroup
. DefiningIdealOfMiddlePartOfUnitaryGroup
(see comments in code)
removed misplaced description in a grouped operation
removed obsolete @Returns at grouped operations
@Group at the bottom
renamed DefiningIdealOfUnitaryGroupModuloIdeal -> DefiningIdealOfUnitaryGroup
cited Serre05 in docu of SerreCharacteristicSubgroup
grouping of docu in UnitaryGroup.gd
declared and installed DefiningIdealOfMiddlePartOfUnitaryGroupUsingAdaptedBasis