Multiplication of a sparse vector with a scalar

Hi,

Thanks for the very useful library.

It seems like the library is missing a function of a vector-times-scalar multiplication.
Perhaps something like this:
```haskell
vecMult :: Num a => a -> SM.SparseVector a -> SM.SparseVector a
vecMult k v = SM.intersectVecsWith (\x _ -> k*x) v v
```

It would also be nice to have fromAssocList and toAssocList functions for sparse vectors, and not only for sparse matrices.

Cheers,
Jan