utility::matrix add function could be operator+=

[Peter]

The functions below look pretty much like operator+=, don't they?

int add (const matrix &b)
int add_constant (const double d)

[Jari Häkkinen]

Removing type "idea" and changing this to a "request".

[Peter]

Also scale could be const matrix & operator*=(double)

[Jari Häkkinen]

(In [773]) Fixes #138. Removed add, sub, scale and added appropriate operators.