[Single/Double.Matrix] Remove redundant calculation of abs in frobenius norm

The formulas found in various references do not have abs. Furthermore,
when multiplying A*A^T or A^T*A (for the frobenius norm we only care
about the diagonal so it doesn't matter which product is used) the
i-ith diagonal element of the final matrix comes from the multiplication
of line i with itself. Therefore, all numbers involved will be multiplied
with themselves resulting in numbers that are always non-negative.

Signed-off-by: Alexander Karatarakis <alex@karatarakis.com>

src/Numerics/LinearAlgebra/Double/Matrix.cs

@@ -99,7 +99,7 @@ namespace MathNet.Numerics.LinearAlgebra.Double
             var norm = 0.0;
             for (var i = 0; i < RowCount; i++)
             {
-                norm += Math.Abs(aat.At(i, i));
+                norm += aat.At(i, i);
             }
 
             norm = Math.Sqrt(norm);

src/Numerics/LinearAlgebra/Single/Matrix.cs

@@ -99,7 +99,7 @@ namespace MathNet.Numerics.LinearAlgebra.Single
             var norm = 0.0f;
             for (var i = 0; i < RowCount; i++)
             {
-                norm += Math.Abs(aat.At(i, i));
+                norm += aat.At(i, i);
             }
 
             norm = Convert.ToSingle(Math.Sqrt(norm));