Improve implementation for parsing Lizard reports

Changes within this pull request improve the parsing of Lizard complexity reports:

Ensure compatibility with SonarQube 7.0 by removing use of methods that have been removed from the scanner on the server side (solves #56).
Remove use of deprecated metrics (solves #54).
Reduce technical dept by improving readability, annotating methods with their null-ability, use updated API:s and reduce usage of deprecated classes.
Warn instead of throwing an exception when the lizard report XML file is not available.

Backelite / sonar-objective-c