refactor mlab spectrum, window, and detrend functions to improve modularity, testability, and performance
implement tests for refactored mlab spectrum, windowing, and detrend functions, with 100% coverage
add more spectral plots and add new spectrum types to specgram in _axes
add tests for new spectrum types in _axes
add demo for new spectral plot types
added information regarding new spectrum functions to CHANGELOG, whats_new, and api_changes