Prevent overflow of matrix interation values

[sangmo-kang]

Dear Maintainer.

When creating a matrix, if the maxLinesPerFile exceeds 65535, the matrix size is determined by the overflowed value.
This can cause a segmentation fault when initializing the matrix using fill.
Also, when comparing two files, if the product of the number of lines in both files exceeds 4294967295, there is a possibility to access the index using the overflowed value.
I suggest changing the variable type that can be used as the matrix index from unsigned to size_t.

Thanks.

[dlidstrom]

Hi @sangmo-kang ! Thanks for your contribution. It looks like the build isn't working any longer. I'll see if I can get it to work and then I'll merge your PR.