ENH: Make it as easy as possible to get predictions from a windowed frame classification model

[NickleDave]

• add forward method to WindowedFrameClassificationModel described in ENH: Add abstraction that represents family of models that classify frames / time bins from spectrogram windows #603
• this forward method will have a timebin_dur argument so it can convert vector of labeled time bins / frames to a sequence of intervals with start and stop times + labels, i.e. segments with onset + offset times
• save timebin duration to spectrogram files, so they are handy when loaded
• add convenience function / transform that's easy to instantiate and converts a spectrogram to a batch of consecutive, non-overlapping windows + a padding mask when padding is needed to create the rectangular batch of windows
  • this probably exists as something like EvalTransform already but not sure how easy it is to use outside of vak.core internals