#74 Add function to close context

[Peter Becker]

There should be an option to create an object as
supremum for a nominal scale. E.g. you have the objects
(COL1 = VAL1), (COL1 = VAL2) and (COL1 = VAL3), which
do not cover all possible values of COL1 but the other
values should not be separated in the scale, then you
need (not(COL1 = VAL1) and not (COL1 = VAL2) and not
(COL1 = VAL3)) as object clause on the top element,
i.e. an object like this has to be created not matching
any attribute.

If there is already an object matching no attribute,
this has to be changed instead.