DOC: Elaborate the usage of function objects

Summary

Documentation and usage examples are limited for some of the function objects. This somewhat hampers the efforts of users to access all capabilities of them.

Therefore, we decided to elaborate the usage of function objectcs in:

• Extended Code Guide (comprises the full spectrum of details)
• Header files (comprises the minimal set of info, directing the users to the central info hub, the Extended Code Guide)

In parallel to the Extended Code Guide improvements (corresponding: Extended Code Guide:doc-FOs-part-1), it is useful:

• to provide a minimal documentation in the header files of the function objects
• to provide at least one example of usage per function object in tutorials

In addition, style/implementation consistency across function objects can be imposed for:

• update libs of etc/caseDicts/postProcess items
• ensure destructor=default
• ensure no copy construct/no copy assignment
• static data members
• ensure constness
• change lookupOrDefault() to getOrDefault()

Resolved bugs

#1594 (closed)

Risks

Considerably low.

Regression tests are pending.