add a coverage drop threshold to limit codecov failure spam

[lbergelson]

Codecov is adding failure messages for very small coverage changes that aren't worrisome. I'm adding a threshold that should hopefully fix it so only more significant coverage drops are reported as failures.

[codecov-io]

Codecov Report

Merging #803 into master will increase coverage by 0.01%.

@@             Coverage Diff              @@
##             master     #803      +/-   ##
============================================
+ Coverage     64.73%   64.74%   +0.01%     
- Complexity     7134     7138       +4     
============================================
  Files           525      525              
  Lines         31679    31679              
  Branches       5414     5414              
============================================
+ Hits          20506    20512       +6     
+ Misses         9028     9026       -2     
+ Partials       2145     2141       -4

Impacted Files	Coverage Δ	Complexity Δ
...main/java/htsjdk/samtools/SAMRecordSetBuilder.java	89.11% <ø> (+0.8%)	64% <ø> (+2%)	✅
...a/htsjdk/samtools/cram/encoding/rans/Decoding.java	85.71% <ø> (+3.57%)	12% <ø> (+1%)	✅
...samtools/util/AsyncBlockCompressedInputStream.java	76% <ø> (+4%)	13% <ø> (+1%)	✅

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[droazen]

👍 merge when ready