Support Gaussian elimination for static matrices

[lucaferranti]

Feature description

At the moment Gaussian elimination supports only mutable arrays (because it calls rref). Would be nice to support also static arrays, probably the static matrix should be converted to mutable to use rref.

Minimum working example

A = @SMatrix [2..4 -1..2;-2..1 2..4]
b = @SVector [-2..2, -2..2]
solve(A, b, GaussianElimination)

Additional information