Add oct_ratio option to specify base-2 system or base-10 system

[peterleif]

I added support for base-10 octave ratio to your library. The default behavior is kept to base-2 for backward compatibility. It would be happy if you consider this feature to be merged.

[Nitnelav]

👍 for base 10. I don't know if your filtering implementation includes decimation stages but if so, they'll need to be updated to match the base10 filters.

[peterleif]

👍 for base 10. I don't know if your filtering implementation includes decimation stages but if so, they'll need to be updated to match the base10 filters.

I checked the filtering functions (in sosfiltering.py and sosfilt.c) and they do not include decimation stages. I believe no modification is needed in filtering stage.