Added chan_select.py [issue 4]

Run as python3 chan_select.py. The nds host and port are optional arguments to the script.

Provides a dataviewer like interface to ndscope. A tree structure of the available channels is at the top left. The list of channels to plot is at the top right. Channels can either be dragged from the tree and dropped into the list, or empty rows can be added with the plus button and edited. If a channel in the list does not exist, then its text is turned red. Only channels that are checked are plotted. Rows can be deleted from the list by selecting them and pressing the minus button. Rows can be rearranged by dragging and dropping them. The tree can be navigated by the keyboard arrows, and channels in the tree can be added to the list by pressing enter when they are selected. T0 and T1 are the -t options to ndscope. When the plot button is pressed the ndscope process is started in the background so that multiple instances can be created from the gui.

I have not figured out how to drag and drop channels from medm. I believe they can be copied and pasted, but it is a bit of a cumbersome process. Hopefully once we move to caqtdm this will no longer be an issue.

The code probably needs more comments. I would prefer to wait on that until any major requested changes are done.