Improve getting a Bundle total channel count

Improve getting a Bundle total channel count

|Bundle::nchannels()| creates a ChanCount on demand, by iterating over
the |_channel| member variable. The sum of all |nchannels().n(t)| over
all non-NIL DataTypes |t| is thus equal to |_channel.size()|.

Consequently, calling |nchannels().n_total()| is a convoluted (and slow)
way of getting |_channel.size()|. Add a method |Bundle::n_total()| that
directly returns the latter.