detrends: mask NANs in combined image

We may have a NAN in the combined image if there were no unmasked
inputs to a particular pixel.  The result should have a mask value
of NO_DATA, but the NAN needs to be suppressed so it doesn't
propagate into the image when we do ISR with this detrend.
Merge branch 'tickets/HSC-1030'
detrends: make NAN masking more generic

Adds support for masking NANs in an Exposure and DecoratedImage.
Changed the style of masking to simply replace NANs with the
median using numpy rather than interpolating, as the interpolation
code is specific to an Exposure (it ideally needs a Ccd so it
can tell whether the rows are vertical or horizontal).
Merge branch 'tickets/HSC-1016'
ProcessExposure: allow CCDs to be ignored

The "ignoreCcdList" functionality was removed when the focus
processing was added (since it was a mechanism to ignore the
focus CCDs, but they were now supported).  However, this
functionality is useful for restricting the amount of processing
required in testing.

Added an 'ignoreCcdList' parameter to the ProcessExposureConfig,
allowing individual CCDs (specified by identifier) to be ignored
and some small changes to support this functionality.
Merge branch 'tickets/HSC-1006'