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version 2.1.0, 2016-07-29
- last_o2():	new function
- changed code to use measurements::conv_unit() rather than the defunct birk::conv_unit()
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version 2.0.0, 2016-03-13
- import_o2():		when multiple units of O2 are recorded, the notification has been downgraded from a warning to a message.
- import_o2():		improved speed
- o2_unit_conv():	MAJOR UPDATE: updated so that any O2 unit can be provided as an argument, not just percent air saturation. No longer backwards compatible with v1.0.0.
- o2_unit_conv():	added kPa, ml_per_l, mmHg, and inHg as unit options
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version 1.0.0, 2015-02-07
- import_o2():		new function
- o2_unit_conv():	new function
