Improve code quality of DefaultSolver a bit.

[AntoniusW]

Addresses issue #251.

[codecov]

Codecov Report

Merging #254 into master will increase coverage by 0.01%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master     #254      +/-   ##
============================================
+ Coverage     77.00%   77.02%   +0.01%     
  Complexity     2068     2068              
============================================
  Files           150      150              
  Lines          6784     6781       -3     
  Branches       1187     1186       -1     
============================================
- Hits           5224     5223       -1     
+ Misses         1165     1164       -1     
+ Partials        395      394       -1     

Impacted Files	Coverage Δ	Complexity Δ
...va/at/ac/tuwien/kr/alpha/solver/DefaultSolver.java	62.50% <83.33%> (+0.30%)	59.00 <0.00> (ø)

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[madmike200590]

Looks good to me!