Fix violations found by running JuliaCheck on itself

[reniers-tiobe]

In particular, solved violations for:

• underscore-prefix-for-private-functions. For the methods on Check subtypes, we solved it my making them explicitly extend the methods in Analysis.
• multiline-comments-for-many-lines
• module-end-comment
• use-isnothing-to-check-for-nothing-values

Note that JuliaCheck already runs on itself as part of the tests. The results are found in file test/JuliaCheck-self.out.

[reniers-tiobe]

Submitted RM-37764 for fixing FP on long-form-functions-have-a-terminating-return-statement

[reniers-tiobe]

Submitted RM-37771 for false positive found.

I propose not to await this and merge this PR back.

[brouwers-tiobe]

Skimmed through the changes and did not find anything strange, and tests pass for me locally as well.

Beside that, I ran JuliaCheck on files under ./src and ./checks directories using:

find ./checks/ ./src -iname *.jl | xargs julia --project src/JuliaCheck.jl --output json --outputfile ./selfcheck_out.json --

Most violations for the mentioned rules are indeed gone. The only exception is underscore-prefix-for-private-functions, for which one violation remains in ./checks/PreferConstVariablesOverNonConstGlobalVariables.jl and several in the ViolationPrinter implementations (on the Output implementation methods). The latter are similar to the Check methods extending Analysis, I believe.

The printers directory is not covered by the self check in runtests.jl, so I guess these were not caught. Not sure if we still want to address those; regardless a lot was fixed -> LGTM

[reniers-tiobe]

Note that there are still violations left, but we will leave them for later. Some are believed to be false positives and tickets have been created for them.