The increment after this frame for the angular setting of
the specified axis in degrees. The sum of the values
of _diffrn_scan_frame_axis.angle,
_diffrn_scan_frame_axis.angle_increment and
_diffrn_scan_frame_axis.angle_rstrt_incr is the
angular setting of the axis at the start of the integration
time for the next frame and should equal
_diffrn_scan_frame_axis.angle for this next frame.