Bump minimum scipy and numpy version requirements
Created by: alurban
This PR bumps the minimum scipy version requirement to 1.2.0, and the minimum numpy version requirement to 1.12.0. The former is dictated by a need to support median averaging in spectrograms, and the latter is determined by a desire to support numpy.geomspace, both the most encompassing requirements I could find scattered throughout gwpy.
Per instructions left in comment lines, this includes a somewhat large-scale refactoring of gwpy/signal/spectral/_scipy.py.