Documentation fix on SchemaMapper ctor's confusing behavior.
add NO_DATA mask plane, and use in warping/convolution

The EDGE mask plane was originally intended to mean "pixel near the
edge of the detector, so we haven't been able to properly convolve to
search for sources", but it was hijacked to mean "pixels in a warped
image that are off the edge of the original".  It has since had both
meanings, which should be distinct.

We introduce the NO_DATA mask plane to mean "pixels off the edge of
the original", and reserve "EDGE" to mean "pixels near the edge".
afw::math::edgePixel (used for pixels with no good inputs when
warping) now returns a value for pixels *off* the edge (should it
be renamed? but that's changing a public API).  Convolution now
masks pixels near the edge as EDGE if copying edge pixels, and
NO_DATA otherwise.
tests: not necessary to re-seed mask planes

Re-seeding the mask planes with the wrong order or number than they are
originally set in Mask.cc results in the wrong values when convolving.
tests: update warping to check new NO_DATA mask bit

NO_DATA is now used in warping instead of EDGE.
ds9: display new NO_DATA mask bit as orange
Merge branch 'tickets/DM-669'
Minor doc fixes in afw::table
Added MatchControl and implemented it for matchXY and MatchRaDec

N.b. added some missing tests on matchXY, although it is unchanged by this
changeset
Merge branch 'tickets/DM-855'
New base class for image-associated 2-d functions.
Added 2-d Chebyshev class with new API
Merge branch 'tickets/DM-796'
Add ApCorrMap class in C++.
Fix sign in comparison in ApCorrMap lookups
Make drawLabels with no frame specified write to default display
Implement and test includeMismatches option in MatchControl object
Python wrappers, unit tests for ApCorrMap
Add an ApCorrMap to ExposureInfo (and by extension, Exposure).
Add ApCorrMap to ExposureRecord
Merge branch 'tickets/DM-797'