Documentation on window

Key: windows
	The use of "select=window" is used to select a subset of spectral
	windows (or IF bands in AIPS terminology) in a uv data-set. For
	example, to select the second and third spectral windows, use

	  select=window(2,3)

	Some explanation is needed for the interaction between "select=window"
	and the line keyword.

	Window selection complements velocity linetypes (line=velocity or
	line=felocity). In this case only data from the selected windows
	is used in determining the velocity channels.

	There are some limitations when using window selection with channel
	linetype ("line=channel"). Firstly the selected windows must
	be contiguous (i.e. you cannot give "select=window(1,4)") when combined
	with "line=channel" selection. Secondly the channel specification
	in the line keyword is relative to the first channel in the first
	selected window. For example, if you use

	  line=channel,10,1,1 select=window(2,3)

	you will be get 10 channels starting from the first channel in window
	2. If windows 2 and 3 had 8 channels each, then you would get all
	8 channels from window 2, and the first two channels from window 3.