Fix CallGraphAlgorithm.addClass(..) implementation

[swissiety]

• in case of non-existing methods in the CG it threw:

Exception in thread "main" java.lang.NullPointerException: Node for <org.apache.poi.ss.formula.functions.FreeRefFunction: org.apache.poi.ss.formula.eval.ValueEval evaluate(org.apache.poi.ss.formula.eval.ValueEval[],org.apache.poi.ss.formula.OperationEvaluationContext)> has not been added yet

• don't fail if nothing new was added (or a subtask was already calculated before)
• return more convenient Signature from JavaSootClass.getType()

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (129eeb7) 63.78% compared to head (54191f1) 63.78%.

Files	Patch %	Lines
...a/sootup/callgraph/AbstractCallGraphAlgorithm.java	63.63%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #783      +/-   ##
=============================================
- Coverage      63.78%   63.78%   -0.01%     
- Complexity      3385     3386       +1     
=============================================
  Files            315      315              
  Lines          15017    15022       +5     
  Branches        2534     2535       +1     
=============================================
+ Hits            9579     9582       +3     
  Misses          4545     4545              
- Partials         893      895       +2     

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[JonasKlauke]

This is only true for CHA algorithm or for the RTA (if the given classtype is instantiated)
If we want to actually support this function the abstaractcallgraphalgorithm has to be abstract and this method has to be developed for each algorithm.

additionally if this class was already in the view all edges are already included.
I think the only reason for this method if you want to force the call graph to contain the class, even when the algorithm would say it is not contained, and then we could do it like this (like CHA)