Make vulnerabilityType more descriptive

[volivan239]

By now, for each analysis there is only one produced vulnerabilityType. However, there are analyzers that produce more than one type of vulnerability which should be reflected in report.

• UnusedVariableAnalyzer should produce something like unused variable, unused parameter and unused return value
• NpeAnalyzer can also produce something like redundant null check.

We should rework these analyzers (and maybe some interfaces) so that analyzers could easily produce different type of vulnerabilities