Better parser errors.
Accept anything for cells in parseNodeEdges.

Fixes afternoonskyscrapers parse error.
Get rid of 'Ref', redundant with 'Alpha'.
Edges as pairs of adjacent cells.
Simplify parsing of afternoon skyscrapers.
Determine Sudoku size from map keys.
Make edge parsing independent of grid size.
Parse to full grids for RefGrid, as well.
Get rid of grid size, parsing and data.

The rendering still uses the size for the moment.