New Features:
    Two more types of "negative" chips: DUP and SPLT
    (an introductory level with explanation will be added later).

Bugfixes:
    Number of negative chips in "endless" stages reduced.
    Revised the calculation of stage difficulty in endless stages.
    Isolated slots are avoided, paths have a minimum length.

Miscellaneous:
    Removal cost of negative chips depend on how many have already been removed.
