Simplify generation of test matrix in testRandGmres.cpp example

[pelesh]

Consider simplifying/documenting better the logic for generating sparse matrix for testing randomized GMRES solvers in function generateMatrix. It is hard to parse what that function does and why.

Also, consider simplifying if statement in that function

if (((!c) && (((j - rowptr[i]) * N/nnz_per_row + (N%(N/nnz_per_row))) >= i)) || ((!c) && (j == (rowptr[i+1] - 1)) )) {

with something like this:

if ((!c) && (((j - rowptr[i]) * N/nnz_per_row + (N%(N/nnz_per_row))) >= i)) || (j == (rowptr[i+1] - 1)) ))

(same meaning, more readable) Maybe even nest the or inside the and

Originally posted by @shakedregev in #218 (comment)