Compute and propagate statistical uncertainty in live time faction computation

[paolomassa]

To get a better estimate of the uncertainty we have on the recorded counts, we should compute and propagate the error on the live time fraction. This involves both imaging and spectroscopy, and is particularly relevant when the live time fraction is low.

[grazwegian]

A rough estimate is currently made for spectroscopy using the total error on the triggers.

STIX-GSW/stix/idl/processing/livetime/stx_spectrogram_livetime.pro

Lines 94 to 98 in bb8acc8

	corrected_counts_upper = spectrogram.counts/livetime_fraction_upper
	corrected_counts_lower = spectrogram.counts/livetime_fraction_lower
	error_from_livetime = (corrected_counts_upper - corrected_counts_lower)/2.
	corrected_error = sqrt( (spectrogram.error/livetime_fraction)^2. + error_from_livetime^2.)