DownmixMono channels_first wrong default value

[ptrblck]

While the docs state the default value for channels_dim is True, it seems to be set to None instead as shown here.

This would yield to the default channel dimension as dim1:

channels_first = None
ch_dim = int(not channels_first)
print(ch_dim)
> 1

Reported in the forum here.

If you want, I could create a fast fix using channels_first=True as the default value or update the docs instead.

Best,
ptrblck

[jamarshon]

Removed DownmixMono here #152