Refactor the code to reduce package scope

Major refactoring of the code is needed to reflect that the package is meant only for seismic data analysis now (bigger scope of signal processing and control systems modelling is now filled by Spicypy)

• Reduce complexity everywhere by assuming we only ever need to deal with seismic data, and heavily rely on obspy package to process it
• For spectra calculation, use existing functionality in Spicypy; add easy conversion handles to Spicypy FrequencySeries and TimeSeries
• Update package description and docs to reflect that