Use enum instead of string for default initial phase

[rtaupe]

No description provided.

[codecov]

Codecov Report

Merging #253 into phase_saving_and_restarts will decrease coverage by 0.12%.
The diff coverage is 66.66%.

@@                       Coverage Diff                       @@
##             phase_saving_and_restarts     #253      +/-   ##
===============================================================
- Coverage                        76.75%   76.62%   -0.13%     
+ Complexity                        2148     2123      -25     
===============================================================
  Files                              155      155              
  Lines                             7102     7050      -52     
  Branches                          1234     1213      -21     
===============================================================
- Hits                              5451     5402      -49     
+ Misses                            1246     1245       -1     
+ Partials                           405      403       -2     

Impacted Files	Coverage Δ	Complexity Δ
...pha/solver/heuristics/PhaseInitializerFactory.java	43.47% <0.00%> (-2.36%)	2.00 <0.00> (ø)
...t/ac/tuwien/kr/alpha/config/CommandLineParser.java	81.52% <50.00%> (-0.04%)	27.00 <1.00> (+1.00)	⬇️
...ava/at/ac/tuwien/kr/alpha/config/SystemConfig.java	74.46% <100.00%> (+0.55%)	33.00 <1.00> (+1.00)
...va/at/ac/tuwien/kr/alpha/solver/SolverFactory.java	79.16% <100.00%> (ø)	4.00 <0.00> (ø)
.../ac/tuwien/kr/alpha/common/ComparisonOperator.java	63.15% <0.00%> (-5.27%)	4.00% <0.00%> (-1.00%)
.../tuwien/kr/alpha/common/atoms/ExternalLiteral.java	88.88% <0.00%> (-2.03%)	19.00% <0.00%> (-26.00%)
...alpha/common/atoms/FixedInterpretationLiteral.java	100.00% <0.00%> (ø)	1.00% <0.00%> (-1.00%)
...in/java/at/ac/tuwien/kr/alpha/common/Interner.java	100.00% <0.00%> (+10.00%)	4.00% <0.00%> (+1.00%)

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