Improve implementation for parsing Lizard reports

[raatiniemi]

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 Add sonar 7.0 compatibility #56).
• Remove use of deprecated metrics (solves Lizard scanner - Complexity / File measure exception for each file  #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.