Fix index overflow in quickPartition methods.

[bkmacy]

When sorting a large array where the length is more than half the maximum integer value, one of the computed indices in quickPartition may overflow, which can lead to a negative array index. This patch fixes the overflow issue using longs to compute the index value.

[codecov]

Codecov Report

Merging #17 (38bcf61) into master (d8ec668) will decrease coverage by 0.00%.
The diff coverage is 16.66%.

@@             Coverage Diff              @@
##             master      #17      +/-   ##
============================================
- Coverage     40.85%   40.84%   -0.01%     
- Complexity     7221     7222       +1     
============================================
  Files           264      264              
  Lines         63167    63167              
  Branches      10299    10299              
============================================
- Hits          25809    25803       -6     
- Misses        35659    35663       +4     
- Partials       1699     1701       +2     

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/edu/mines/jtk/util/ArrayMath.java	59.11% <16.66%> (ø)	1667.00 <0.00> (ø)
core/src/main/java/edu/mines/jtk/util/RTree.java	77.25% <0.00%> (-0.35%)	30.00% <0.00%> (ø%)
core/src/main/java/edu/mines/jtk/mesh/TriMesh.java	60.83% <0.00%> (-0.23%)	243.00% <0.00%> (-1.00%)
core/src/main/java/edu/mines/jtk/mesh/TetMesh.java	51.43% <0.00%> (ø)	388.00% <0.00%> (ø%)

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