NUMBERS-193: Add a double-double (DD) number

[aherbert]

Add a double-double extended precision floating-point number.

This adapts the DD class from Commons Statistics Inference as a general use number.

[codecov-commenter]

Codecov Report

Merging #137 (fab2444) into master (c822a02) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #137      +/-   ##
============================================
+ Coverage     99.14%   99.23%   +0.09%     
- Complexity     1683     1813     +130     
============================================
  Files            65       67       +2     
  Lines          4308     4814     +506     
  Branches        856      893      +37     
============================================
+ Hits           4271     4777     +506     
  Misses           10       10              
  Partials         27       27              

Files Changed	Coverage Δ
.../main/java/org/apache/commons/numbers/core/DD.java	100.00% <100.00%> (ø)
...n/java/org/apache/commons/numbers/core/DDMath.java	100.00% <100.00%> (ø)

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