demod: Complex demodulation
In GPPFourier: Calculate Gross Primary Production (GPP) from O2 Time Series

Retreive the time varying amplitude of a signal with fixed carrier frequency by complex demodulation

1	demod(x, dt, w1 = 2 * pi/1, filttype = c("MA", "bw"), Nf, nf = 1, freq)

`x`	Vector containing time series observations at constant sampling interval
`dt`	Time step of time series. Arbitrary units. Class `Numeric` or `difftime`
`w1`	Carrier frequency. Inverse unit of `dt`.
`filttype`	Filter type to be applied for low pass filtering. "MA" for moving average; "bw" for butterworth filter.
`Nf`	Moving average filter width
`nf`	Number of iterative applications of moving average filter. To remove filter bandwidth.
`freq`	Cut-off frequency of Buttersworth filter