calc_n_coarse_chan (base_reader.py) should default to 1, not raise error

[Richard Elkins]

When errors are encountered,
let's convert those ValueError exceptions to warnings and return the number of coarse channels = 1.

Input from issue #201

[Richard Elkins]

@telegraphic
In the warning situation (cannot compute a number of coarse channels), the returned value will be 1. This has the effect of grouping all of the fine channels together as a single coarse channel.