Better management of the stop / cancel scanned

Sometime user can do a ctr+C by mistake. And the more it goes the longer scan can take time. A part of already acquired data can be still be used but this is not really foreseen for now.

By default we want to check if there is enough projection and scan. If an NXtomo can be created with at least one frame then we should do it. Remaining can be invalid frames with 'mock up' virtual dataset. Of course a clear warning should be displayed in this case.