Check metric citations

[smoia]

Check that the citation for each metric is the original paper proposing the metric.

[tsalo]

One caught by @rzlim08 in #19 (comment):

phys2denoise/phys2denoise/metrics/chest_belt.py

Lines 142 to 146 in cfced3b

	References
	----------
	.. [1] C. Chang & G. H. Glover, "Relationship between respiration,
	end-tidal CO2, and BOLD signals in resting-state fMRI," Neuroimage,
	issue 4, vol. 47, pp. 1381-1393, 2009.

should be "Influence of heart rate on the BOLD signal: the cardiac response function" also by Chang and Glover in 2009.

Or perhaps both? No reason we can't cite more than one paper...