Added functions to add a scalar double to each element of a dense matrix;
subtract a scalar double from each element of a dense matrix; or subtract
each element of a dense matrix from a scalar double.

Some examples in FSharp 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