Pixel size: Reimplement with focal position

Rework the pixel size to handle any changes on directances between source<->sampe<->detector

• If x_axis moves now the source_sample distance is updated
  • It is based on a x_axis_focal_pos attribute
• tomo_detectors have now it's own source_distance which is updated if a detector_axis is defined
  • It is based on a detector_axis_focal_pos attribute
• tomo_detector have now it's own source_distance (the API sample_detector_distance is only an helper)
  • It is based on a detector_axis_focal_pos, or an offset an tomo_detectors, as before

DEMO_SESSION [1]: tomo3_config.source_sample_distance = 500
             ...: DISTANCE(300)
             ...: tomo3_config.detectors.active_detector.offset_detector_distance = 50
DEMO_SESSION [2]: tomo3_config.detectors.active_detector
         Out [2]: tomodet_tomo3_128p tomo detector:
                  Role
                  ----------------------  ---------------------------
                  state                   READY
                  detector                -> tomo3_128p
                  optic                   -> tomo3_128p_magnification
                  
                  distances
                     source <-> sample    500.000 mm
                     source <-> holder    750.000 mm
                                + offset  50.000 mm
                     source <-> detector  800.000 mm
                  
                  sample pixel size
                     active               625.000 um
                     auto                 625.000 um
                     user                 undefined
                  
                  field of view           80.000 mm