Halve PSQT row data

Use symmetry along vertical middle axis of the board
to reduce the number of parameters.

For instance psqt value of SQ_A5 == SQ_A4 and value of
SQ_F8 == SQ_F1.

This is always true, at least until now nobody came in
with an asymmetric psqt table that worked.

Original patch by Lucas.

No functional change.