fix "variable only occurs once" checker

[quentin]

Fix variable only occurs once checker:

• deterministic source location
• handle multi-head rules properly
• do not report ungrounded variables (already reported by another check)

Still not handling properly the variables in negated conjunctions and aggregates.

[codecov]

Codecov Report

Merging #2415 (12a8abe) into master (2914f7b) will increase coverage by 0.04%.
The diff coverage is 81.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2415      +/-   ##
==========================================
+ Coverage   77.75%   77.79%   +0.04%     
==========================================
  Files         474      479       +5     
  Lines       31164    31391     +227     
==========================================
+ Hits        24232    24422     +190     
- Misses       6932     6969      +37     

Impacted Files	Coverage Δ
src/ast/analysis/typesystem/Type.cpp	47.37% <0.00%> (-0.47%)	⬇️
src/ast/analysis/typesystem/Type.h	50.00% <ø> (ø)
...c/ast/analysis/typesystem/TypeConstrainsAnalysis.h	100.00% <ø> (ø)
src/ast2ram/seminaive/ValueTranslator.h	100.00% <ø> (ø)
src/ast2ram/utility/TranslatorContext.h	100.00% <ø> (ø)
src/include/souffle/utility/SubProcess.h	55.00% <0.00%> (ø)
src/interpreter/Node.h	96.95% <ø> (ø)
src/ram/Variable.h	45.45% <45.45%> (ø)
src/ram/Assign.h	50.00% <50.00%> (ø)
src/parser/parser.yy	93.85% <58.33%> (-1.30%)	⬇️
... and 22 more

... and 4 files with indirect coverage changes