Human Solve Started

Implementing a new human solve algorithm.  This algorithm iteratively
removes possible values from the selectable grid until only one
possibility remains.  It them selects that value and then  continues
selected values this way.  So far, the algorithm gives a good start to
solving the problem, but it needs to consider more multidimensional
removal of possibilities - like if 4 is possible only on the bottom row
of this sub square, so the other sub squares must not have 4 in the
bottom row.