Add test for process1.window
Also test window(1)
Do not throw if window is called with n < 1

This would otherwise throw an UnsupportedOperationException because of Vector().tail:
Process.range(0,5).window(0).runLog.run
Behave similar to chunk and throw a RuntimeException if the window size is not positive.
Merge pull request #82 from fthomas/test-window

Tests for process1.window