nGetMatDim: Compute matrix dimension based on the length of a flattened...
In charlotte-ngs/rgrm:

The flattened vector contains all elements of the lower triangular part of a symmetric matrix including all diagonal elements. Because any row i of a quadratic matrix contains i elements below the diagonal, including the diagonal element, the flattened vector of an n * n symmetric matrix contains the sum of all natural numbers up to and including n. That length has a closed form and corresponds to n*(n+1)/2. Given the length of the flattened vector, we have a quadratic equation for n, that can be solved. We are only interested in the positive solution which is n = (-1 + √(1+8l))/2