Fix IntervalTerm parsing

[madmike200590]

Resolves #292, resolves #324.

• Introduces additional non-terminal symbols in grammar file in order to properly distinguish negative integers (e.g. interval start terms) from negated arithmetic terms (see #292)
• Adds interface IntervalTerm to api module and moves implementation to commons module

[codecov[bot]]

Codecov Report

Merging #341 (0b488fc) into master (6a2975e) will increase coverage by 0.54%. The diff coverage is 46.93%.

@@             Coverage Diff              @@
##             master     #341      +/-   ##
============================================
+ Coverage     70.23%   70.77%   +0.54%     
- Complexity     2123     2150      +27     
============================================
  Files           182      182              
  Lines          8023     8025       +2     
  Branches       1423     1424       +1     
============================================
+ Hits           5635     5680      +45     
+ Misses         2028     1970      -58     
- Partials        360      375      +15     

Impacted Files	Coverage Δ
...at/ac/tuwien/kr/alpha/core/atoms/IntervalAtom.java	64.51% <ø> (ø)
...grams/transformation/ArithmeticTermsRewriting.java	87.34% <0.00%> (ø)
...re/programs/transformation/ChoiceHeadToNormal.java	85.00% <0.00%> (ø)
...ams/transformation/IntervalTermToIntervalAtom.java	92.85% <ø> (ø)
...ava/at/ac/tuwien/kr/alpha/commons/terms/Terms.java	22.91% <7.69%> (-2.80%)	:arrow_down:
...ac/tuwien/kr/alpha/core/atoms/IntervalLiteral.java	68.57% <37.50%> (-7.30%)	:arrow_down:
.../kr/alpha/core/grounder/FactIntervalEvaluator.java	76.47% <44.44%> (-9.25%)	:arrow_down:
...uwien/kr/alpha/commons/terms/IntervalTermImpl.java	33.33% <71.42%> (ø)
.../tuwien/kr/alpha/core/parser/ParseTreeVisitor.java	85.64% <100.00%> (+0.46%)	:arrow_up:
... and 7 more

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[madmike200590]

Looks good, see comments for change requests. Biggest one is probably that we construct negative integers directly in the lexer, not parser.

Thanks for the feedback! It's all taken care of, please take another look.