Add ability to change the index

[bparks13]

Currently there is no way to record data from more than Onix system. Add an index to the editor, so that it can be specified and multiple onix-source plugins can be used.

[jonnew]

Per Aaron -- If the default index is used, then you can open multiple systems but you cannot select which one you are talking to explicitly. This is how the Acquisition Board works currently. For now, we can use this by using index -1 instead of 0.