Add new metric for test/code ratio

[denniebouman]

Add new metric for test/code ratio (the ratio between the test code and the production code).

Describe the solution you'd like
This metric should give a ratio between the test code and the production code. Based on CLOC, see also #460
Sources should (or could) be:

• CLOC output file for productioncode
• CLOC output file for testcode

See also: https://github.com/AlDanial/cloc

Tasks:

• New metric: "Test code ratio"
• One supporting source: CLOC
• When CLOC is used as source for test code ratio it should have two URL parameters, one for the JSON with the test code side and one for the JSON with the production code size. See Jenkins.py in the data model sources folder for an example of how to override the label and popup of the default URL parameter.
• Add test code ratio collector for CLOC.
• Update changelog
• Check whether we can use SonarQube as well: it needs to give information per file: path, filename, size, and language: unfortunately SonarQube has no file size for test files, at least not in all languages, so SonarQube support is not possible.