calc_psii: Calculate functional PSII content
In KeachMurakami/kagari:

. This function calculates functional PSII contents according to Chow et al. (doi:10.1071/ PP9910397).

1 2	calc_psii(calb, positive, negative, amp, leaf_area, hz = 10, e_per_o2 = 4, artefact = options()$heat_artefact_psii)

`calb`	signal amplitude corresponding to 1 mL O2 which was injected for the calibration, unit: grid.
`positive`	a slope corresponding to net O2 evolution rate under repeatitive single-turnover flashes and weak far-red light, unit: grid min^-1.
`negative`	a slope corresponding to O2 uptake rate under weak far-red light, unit: grid min^-1.
`amp`	an expansion coefficient, no-unit.
`leaf_area`	an area of the enclosed leaf in the chamber, unit: m^2.
`hz`	a frequency of single-turnover flash, unit: s-1, default: 10.
`e_per_o2`	a number of electron consumed in PSII to evolve one O2, unit: mol mol^-1, default: 4.
`artefact`	a factor to correct non-photosynthetic signal, i.e. heating by irradiance, unit: grid min^-1.