Action fix

[tonyday567]

Addressing #114 but called scalars Actor pretty much following the subhask line.

I'm not sure this is the optimal approach, however, and need to test a variation. At the moment, you can only specify one instance eg

type instance Actor (Matrix m n a) = a

instance (Multiplicative a) => Actor (Matrix m n a) where
  (*.) a h = fmap (a*) h

but this then precludes defining another instance eg

type instance Actor (Matrix m n a) = Vector m a

instance (Multiplicative (Vector m a)) => Actor (Matrix m n a) where
  (*.) = ???

I feel as if this should be (just) possible, but didn't get close on a first attempt.