Visibility creation and calibration improvements

[samaloney]

• Support other pixel combinations, currently only works with large pixels only, small pixels only, small and large, top bottom.
• Check use of phase_center v centre make sure correct and keep same nomenclature as xrayvision
• Don't have to overwrite u and v coordinates

[paolomassa]

Check also units of the STIX visibilities

[paolomassa]

Also, I think that the output of create_visibility and of calibrate_visibility should be a visibility structure. Maybe this is also related this issue in xrayvision