add some options and refactor

[Saurabh7]

@karlnapf I was trying to add FITC inference to GPregression but I don't quite understand how allowing the user to set inducing points should work?

should one be allowed to set any point on the X-Y grid as inducing point or only on the x-axis? (Y axis are regresison labels here)

could you give me a quick gist about this?

[karlnapf]

left click adds regression point right click adds inducing point

makes sense?

[karlnapf]

patch looks good ill check locally now