Dense vector cast

Hi David,

I have a pretty big speed drain in the convolve() function for Int, due to casting Int to Long using .map(_.toLong)
I've programmed a cast in DenseVector, which is OK in terms of speed, but I think the reflection is kind of clunky.
Could you comment?

Based on your comments, I'd like to fill in all the cast functions for DenseVector (and perhaps DenseMatrix)

Sincerely,
Kenta