-Throw a UserException if a Locus or ActiveRegion walker is run with -dcov < 200,
since low dcov values can result in problematic downsampling artifacts for locus-based
traversals.
-Read-based traversals continue to have no minimum for -dcov, since dcov for read traversals
controls the number of reads per alignment start position, and even a dcov value of 1 might
be safe/desirable in some circumstances.
-Also reorganize the global downsampling defaults so that they are specified as annotations
to the Walker, LocusWalker, and ActiveRegionWalker classes rather than as constants in the
DownsamplingMethod class.
-The default downsampling settings have not been changed: they are still -dcov 1000
for Locus and ActiveRegion walkers, and -dt NONE for all other walkers.
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