Added operations to add subtract scalar to dense matrix

Operations to add subtract scalar to dense matrix.
Some examples in F# below -

open MathNet.Numerics.LinearAlgebra.Double

let a = vector [0.0 .. 0.1 .. 1.0]
let b = a + 7.1
let c = 7.1 + a
let d = b - 7.1
let e = 7.1 - c

let p = DenseMatrix.init 3 3 (fun i j -> float (i + j))
let q = p + 10.1
let r = 10.1 + p
let s = 10.1 - p
let t = p - 10.1