The original version of AverageAlgorithm.py seemed to be written
to process the Controller's `--outchannels`. It "worked" for several
years because `--outchannels` defaulted to `--inchannels`. But what
we actually always wanted was to process `--inchannels`.

Mostly this was just a matter of semantics, but when I recently added
the ability to average fewer than the full complement of inputs, it
was also necessary to change the `can_produce_data()` method to check
for "any" instead of "all" channels, which in turn required that the
AverageAlgorithm class properly instantiate its `_inchannels` and
`_outchannels` class variables, instead of just set them to `None`.

To briefly explain the different changes in this commit:

- added `Algorithm.__init__(self, inchannels=[channel])` to ensure that
  can_produce_data() would work when run via programmatic interface.
- added `Algorithm.configure(self, arguments)` to AverageAlgorithm.configure()
  to ensure that can_produce_data() would work when run via Controller.py.
- changed all variations of `outchannel` to `inchannel`
- cleaned up where class variables were modified inside process() method