Spectral decomposition and high gamma duplicate code and processing

[emilyps14]

1. processing_data.high_gamma_estimation recomputes the spectral decomposition bands, even if they have already been computed in processing_data.spectral_decomposition. It would be nice if the code would check to see if they have been run already, and use the precomputed bands if possible. If it is computing new bands, it would make sense to append these to the existing decomposition table (or make a new decomposition table?) to save the processing.

2. The spectral decomposition code is copy and pasted between processing_data.spectral_decomposition and processing_data.high_gamma_estimation: https://github.com/catalystneuro/ecogVIS/blob/a5eda733ce7c8f0ed2a2e22c97c4ccaa3b81f00c/ecogvis/signal_processing/processing_data.py#L377-L397 https://github.com/catalystneuro/ecogVIS/blob/a5eda733ce7c8f0ed2a2e22c97c4ccaa3b81f00c/ecogvis/signal_processing/processing_data.py#L467-L488

(I can look into writing a PR for this when I have some time)