Fix recalculate batch step when variables removed

Fix CCOLAMD base cases for 0 and 1 variables

In both cases there's no need to find out any ordering at all: - For 0 variables, an empty Ordering is returned. - For 1 variable, an Ordering with that 1 variable is returned.

Remove unused indices in batch step

In the batch step (in recalculate) we need to remove the unused indices from the variable index, otherwise the elimination would throw an exception saying:

"Requested to eliminate a key that is not in the factors"