Change filter slicing attribute 'nodelay' to 'validsame' since the former can be
confused with the unrelated filter phase delay.
Add decorator to SweepSpectraCython filter function, as it should have been already.
Fix convolution slicing for 'valid', 'same', and 'validsame' so they match numpy/scipy's
convolve when the filter is longer than the array being filtered.
Enable Doppler filtering with N < len(h) for the SweepSpectra implementations. Also
enable embedsignature for Cython functions.
Add inverse filter functions for creating an approximate inverse filter of a specified
length.
Add mistakenly missing copyright/license header to relevant files.