Use a logarithmic scale in spectrograms

[ziotom78]

A linear color scale does not help in viewing PWR/DEM spectrograms, as the lowest frequencies saturate the color scale. It would be better to show log10(Sxx) instead of Sxx.

[fincardona]

Fixed with #17

[fincardona]

Ok but my branch is actually 14 commits ahead of lspestrip:master. Do you know why?