Write MCA tags based on the specified sound field.

Write MCA tags based on the specified sound field.

I had previously assumed that one should write MCA information into
the sound MXF based on what channels actually contain sound rather
than silence.

However a previous example of a stereo DCP gives a verification error
in EasyDCP (see DoM bug #1935) which is solved by adding MCA tags
for each channel in the specified sound field (e.g. at least 6 tags
for a file which is marked as 5.1) even if the audio is really stereo.

This commit removes the "active channels" stuff and makes sure
that if we say a file is 5.1 we write at least 6 MCA tags (and
similarly for 7.1).