Remove default check for is_set

[ClemensLinnhoff]

Reference to a related issue in the repository

#51
also fixes #52

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Remove automatically checking for is_set. Now only explicit is_set rules are checked. Also, is_valid does not generate a warning for a rule violation of a subfield. This is somewhat redundant.

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@jdsika

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• My code and comments follow the contributors guidelines of this project.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation for osi-validation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests / travis ci pass locally with my changes.

[jdsika]

Don't you think that we have to explain how to work with these rules?

[ClemensLinnhoff]

The explanation of how to write rules is in "writing-rules.adoc". Here I added some explanation of the rules that are given as errors in the example trace file and removed the rest.

For the future, I would create a new trace file containing more examples of rule violations and then explain them here. But I would do that after the planned major rework of osi-validation.

[jdsika]

Did you update the osi version?