Needs to parse input for 16 by 16 puzzles & beyond

[pythagoraskitty]

Currently, input is somewhat easily entered and parsed for 9 by 9 puzzles as vectors of strings, and the program interprets any character (including spaces) other than '1' through '9' as an empty cell, but takes advantage of cell entries being single-digit. Hence, parsing 16 by 16 and larger puzzles needs a different approach, such as requiring input to delimit entries with some designated character(s).