Add into() method to Operation.

The into() method is a method that allows operations to be composed
in a clean pipeline expression instead of inverting the order between
method invocations and class construction.

For example,

io.LoadRaster("foo")
  .into(Multiply(_, 3))
  .into(...)

instead of Multiply(io.LoadRaster("foo"))
Merge pull request #468 from joshmarcus/topic/into

Add into() method to Operation.