Default qspace bins number

Merge !72 (merged) first to get a clean diff as this MR builds on it.

This MR adds default values for the number of bins in the QSpace dialog using @crichter solution.

For 18 eta angles [34.07-34.42] with maxipix images, it set default number of bins to qx: 92, qy: 530, qz: 531 Does that sounds correct?

closes #44 (closed)