We should be able to override Waveform Generation with specific channels

This is useful for live viewing.

High level, we want to be able to have a live view of a specific channel where the NI card only plays a specific channel. By default, the WaveformGenerator only takes channel specifications from the config. We want to be able to override this (sometimes) to put the NI card in a state where it's playing waveforms only for the specified channel.

old:
````python
def generate_waveforms(cfg, plot: bool = False):
pass # stuff here.
````
new:
````python
def generate_waveforms(cfg, plot: bool = False, channels: list[int] = None):
pass # stuff here.
````