The value reported by the monitor detector should be given in
_diffrn_scan_frame_monitor.monitor_value.
The value is typed as float to allow of monitors for very
intense beams that cannot report all digits, but when
available, all digits of the monitor should be recorded.
For convenience in automated validation, the deprecated
_diffrn_scan_frame_monitor.monitor_value
is given as an alias, but should be avoided for new data sets.