ENH: Add option for logarithmic y axis and colorscale in magspect() function

[KCollins]

Add 2 separate flags:

• Make axis logarithmic
• Make colorscale logarithmic (include a colorbar!)

Possible additional feature: black and white colormap outside a frequency range of f_lower to f_upper, with the area inside that range colormapped with, e.g., viridis as a form of highlighting. (Implementation: Plot with bw colormap and then overplot only the part of the spectrogram in the range of interest).