Feedback on parameters

Widgets:

• Autobk widget: Rbkg and kmin must be float
• XFTF widget: Kmin, kmax, dk, dk2 must be float

Re-define parameters:

• Noise: Make Estart and Eend relative to the Edge energy. Default values can be the same as for norm range in autobk.
• XFTF and Autobk: k_min defaults to 3, k_max defaults to 0.9*max(kvalues)