Speed up inference in IntelliJ for breeze.linalg.package.scala

The type inference in IntelliJ for breeze.linalg.package.scala was taking many seconds per change. Making the return types explicit fixes it on my system.

Not a problem for `breeze` itself, but I hope you're OK with this change (I guess it's recommended style, especially for :Unit functions)... if no objections, I'll do this for DenseVector/DenseMatrix as well, where I have the same problem. Can't hurt the compile time issue either...