Added IMA read differential support to pstat

[Vb2341]

This feature adds functionality to look at statistics computed from differences between reads of a ramp. Since the IMA reads are cumulative averages, they do not clearly represent a single read of the ramp. By multiplying each read by the exposure time and subtracting adjacent pairs of reads (and dividing by the integration time of that read), a countrate can be computed for each read. This information is much more useful for analyzing trends in background measurements (i.e. detecting variable background from He/Earth limb). This does not fully subtract reads of an IMA from each other, but subtracts the measurements pstat makes (i.e. the midpoint of a read/section of a read).