dmsubs change

Replaces the old version of dmsubs with one that generates a string which represents the polynomial, then evaluates this string. Coefficient values are NOT placed into the string to avoid rounding errors. For example, if the polynomial is `1.532x^2 + 5x - 1`, then the string generated is (simplified for a univariate scalar) `coef(1).*x.^2 + coef(2).*x + coef(3)` which is evaluated for the particular coefficients and x values.