(Three Phase) Variable Frequency and Amplitude

[samderanova]

To operate the LIM at optimal efficiency, we'll need to be able to support variable frequency and amplitude. We'll need to update the run_one_inverter function (or a parent wrapper) to accept frequency and amplitude as parameters and then scale the PDM wave accordingly.

[taesungh]

Let's make this a priority again to unblock #24. Should be as simple as rescaling the sine wave being sampled by multiplying the desired relative amplitude.