The LeastCandidatesCell strategy calculates the number of seemingly valid
candidates (of course, for a problem with a unique solution, there is only
one strictly valid candidate for each cell - a candidate is deemed
'seemingly valid' if it isn't blatantly contradicted by another cell in
that row, column or subgrid) for each cell in the grid, and fills the cells
with the least number of possible candidates first.
The LeastCandidatesNumber strategy calculates, for each combination of
number and sector (where a sector is a generic term that covers rows,
columns and subgrids), the number of valid candidate cells and fills
the the sectors with the least number of possible candidate cells first.