calc_gross_o2: Calculate Pg
In KeachMurakami/kagari:

. This function calculates gross photosynthetic rates (Pg) from net O2 evolution and dark O2 uptake rates.

1	calc_gross_o2(calb, posisitve, negative, amp, leaf_area, artefact)

`calb`	signal amplitude corresponding to 1 mL O2 which was injected for the calibration, unit: grid.
`negative`	a slope corresponding to dark O2 uptake rate, unit: grid min^-1.
`amp`	an expansion coefficient, no-unit.
`leaf_area`	an area of the enclosed leaf in the chamber, unit: m^2.
`artefact`	a factor to correct non-photosynthetic signal, i.e. heating by irradiance, unit: grid min^-1.
`positive`	a slope corresponding to net O2 evolution rate, unit: grid min^-1.