Create ClassGraph

See merge request !46

Resolve "Slicer.java flag -l fails"

Closes #42

See merge request !47

Small fixes for future support of nested calls

See merge request !45

* CallGraph, VariableVisitor: unify visit arguments behaviour to avoid lookup errors of calls
  by the CallGraph that were not registered by VariableVisitor.

Add CI tests on merge

See merge request !44

Implement criterion which don't need a file.

See merge request !43

Add module-info.java to sdg-core

See merge request !42

The PPDG makes some transitive CDs direct, in order to
remove some of them. The ES-SDG then stops some of these,
which had the consequence of not reaching `Enter` in some
cases.

* Removed null pointer when type is not specified
* Improved usage message
* Clarified error messages

# Conflicts:
#	sdg-core/src/main/java/tfm/graphs/CallGraph.java
#	sdg-core/src/main/java/tfm/graphs/cfg/CFGBuilder.java
#	sdg-core/src/main/java/tfm/graphs/sdg/MethodCallReplacerVisitor.java
#	sdg-core/src/main/java/tfm/graphs/sdg/SDG.java
#	sdg-core/src/main/java/tfm/graphs/sdg/sumarcs/NaiveSummaryArcsBuilder.java
#	sdg-core/src/main/java/tfm/nodes/type/NodeType.java