Zombie group on entity creation if an exception is thrown

[gicmo]

As @stoewer discovered during pr #530: When we create a new entity (e.g. BlockHDF5), the parent (e.g. FileHDF5) creates the group, then passes this group to the BlockHDF5 constructor which initialises it (sets attributes, etc.). In the (unlikely, but not impossible) case that an exception gets thrown the group is not clean up after.