code: public int getLengthDirectional(int positionStart, int positionEnd, String startingChain) {
  		int count = getLength(positionStart,positionEnd,startingChain);
  		if(positionStart <= positionEnd) {
  			return count;
  		} else {
  			return -count;
  		}
  	}
docs: Calculates the number of residues of the specified chain in a given range.
  Will return a negative value if the start is past the end.
  @param positionStart index of the first atom to count
  @param positionEnd index of the last atom to count
  @param startingChain Case-sensitive chain
  @return The number of atoms from A to B inclusive belonging to the given chain