Fixed the handling of D tokens.
Fixed the handling of D tokens.

In the SMARTS language, the difference between X and D should be that
one counts only explicit and the other also implicit hydrogens. This
was not the case in the BALL implementation. In addition, we sometimes
counted negative numbers of implicit hydrogen atoms.

tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball,tkemmer/ball