Add comment for units of Syringe Areas

[virajshah]

Line 876 in the main file - If you want to add a syringe that is not one of the BD syringes, you must edit lines 874, 875, 876. Line 876 has self.syringe_areas but I don't know what the units are. mm^2? I'd add an inline comment with the units, for example,

self.syringe_areas = [17.34206347, 57.88559215, 112.9089185, 163.539454, 285.022957, 366.0961536, 554.0462538] # mm^2

(Might also be helpful to add an assert statement to ensure that all three array lengths are the same since someone who wants to add a syringe has to edit all three lines.)