Additional ground nogoods generated for choice rules

[rtaupe]

This is an alternative approach to #282 which aims for the same goal. In comparison, this pull request is (at least currently) less cleanly implemented, but is more efficient (because ground binary nogoods are directly created instead of generating constraints which lead to larger nogoods).

For a discussion, see #282.

[codecov]

Codecov Report

Merging #284 (c468c80) into master (1fbbbf0) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #284      +/-   ##
============================================
+ Coverage     77.46%   77.48%   +0.02%     
- Complexity     2312     2316       +4     
============================================
  Files           178      179       +1     
  Lines          7782     7792      +10     
  Branches       1352     1353       +1     
============================================
+ Hits           6028     6038      +10     
  Misses         1334     1334              
  Partials        420      420              

Impacted Files	Coverage Δ	Complexity Δ
...uwien/kr/alpha/common/NegativeChoicePredicate.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)
...t/ac/tuwien/kr/alpha/grounder/NoGoodGenerator.java	98.48% <100.00%> (+0.12%)	24.00 <0.00> (+1.00)
...ha/grounder/transformation/ChoiceHeadToNormal.java	85.00% <100.00%> (ø)	7.00 <0.00> (ø)

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