Add a JUnit parser

[uhafner]

Adds a parser for JUnit reports that stores the test results in class nodes and provides a new metric TESTS to count the number of tests for a package or class.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (de84cf4) 95.52% compared to head (1297501) 95.64%.

❗ Current head 1297501 differs from pull request most recent head 111cd9c. Consider uploading reports for the commit 111cd9c to get more accurate results

Files	Patch %	Lines
...ava/edu/hm/hafner/coverage/parser/JunitParser.java	95.52%	1 Missing and 2 partials ⚠️
...rc/main/java/edu/hm/hafner/coverage/ClassNode.java	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #53      +/-   ##
============================================
+ Coverage     95.52%   95.64%   +0.12%     
- Complexity      621      677      +56     
============================================
  Files            23       26       +3     
  Lines          1497     1654     +157     
  Branches        209      229      +20     
============================================
+ Hits           1430     1582     +152     
- Misses           38       39       +1     
- Partials         29       33       +4     

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