Work on tiling functions

Created test for quad interpolation; i.e. the interpolation
of 4 tiles to 1 tile at 2x coarser resolution. The 1 tile
has the same bounds as the 4 tiles (i.e. the 4 tiles are
arranged 2x2). The interpolation includes smoothing, to
avert data aliasing at coarser resolutions.

Also changed an implementation detail of the bilinear
interpolation routine. Caller no longer supplies callback to
fill in null data values. Caller must now initialize the whole
data array prior to filling with the interpolation. This may
change further.