Expose expected sample rate in models

[sdatkinson]

This Issue is for models to expose a getter, .GetExpectedSampleRate(), so that interfacing code can understand what the ML models implicitly expect to receive.

Since get_dsp is part of this repo, we are in charge of deciding what to do if no sample rate is recorded as part of the model (i.e. from .nam files pre-sdatkinson/neural-amp-modeler#284). In this case, I will return -1.0, and this will be defined to mean "We don't know." (NeuralAmpModelerPlugin will take this to mean that it must be the implicit default from earlier, 48kHz; others are free to do as they'd like, though this will likely be what they want to do as well assuming they're building for the same user ecosystem.)

The semantics are also that this is the expected sample rate--there's nothing artistically wrong with disregarding it, and this is an artist's tool at the end of the day 🙂