Set the spectrometer proton frequency of the experiment.
id: The experiment identification string to set the frequency of.
frq: The spectrometer frequency. See the `sfrq' parameter in the Varian procpar file or the `SFO1' parameter in the Bruker acqus file.
units: The units of frequency.
This allows the spectrometer frequency of a given experiment to be set. The expected units are that of the proton resonance frequency in Hertz. See the `sfrq' parameter in the Varian procpar file or the `SFO1' parameter in the Bruker acqus file for the exact value.