stop sending irrelevant information to backend

[leofang]

Currently only intensity frames and scan positions are read from H5 in the backend, and the rest is read from the GUI. Therefore, it is possible that we are asking for more info than we actually need for completing the reconstruction.

TODO:

• determine what belongs to "irrelevant information"
• remove those elements from both frontend and backend (related: remove the x/y step size in "Experimental parameters" section of main window #7)
• modify the functions involving reading data from databroker and writing to H5 (related: write the detector x & y directions to H5 #9)

[leofang]

This is related to #74. Swapping to non-default backends like xcale would just ignore the redundant information sent by the frontend. Even our own backend already ignores it...