Improve the end-to-end tests by adding summary tests

[tsaglam]

This PR adds summary tests to the e2e test suite, thus allowing one to quickly glance at the test result and understand the impact of the changes on all tests in total. The information is presented like this:

org.opentest4j.AssertionFailedError: positive deviation over all AVG similarity values:
	count=42, average=0.0260, min=0.0008, max=0.0833

org.opentest4j.AssertionFailedError: negative deviation over all AVG similarity values:
	count=284, average=0.0906, min=0.0012, max=0.2681

This PR also cleans up some code that is touched and improves JavaDoc comments.

[sebinside]

I wonder whether this could lead to false positives due to the floating point nature

[tsaglam]

The test case that uses this only fails if the average of all deltas is above an epsilon value:

JPlag/endtoend-testing/src/test/java/de/jplag/endtoend/EndToEndSuiteTest.java

Lines 170 to 172 in 5a2eeaa

	if (Math.abs(statistics.getAverage()) > EPSILON) {
	fail(textualSign + " deviation over all AVG similarity values:" + System.lineSeparator() + statistics.toString());
	}

This test case is mainly for feedback. For precision, the individual test cases (which produce the deltas) will fail if they deviate by the expected value (again, with a certain epsilon).

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
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66.7% Coverage
0.0% Duplication