feat: max_degree()

[szhorvat]

Please see the chatroom before you comment here.

This adds max_degree(), and is also meant to serve as an example for beginner contirbutors on how to expose a new function.

Why do we want max_degree() instead of just using max(degree())? This function produces 0 instead of -Inf for empty vertex sets, which is convenient when using it as a building block.

There was a documentation item for \dots for degree(), which did not seem to make sense. I removed it.

This is blocked on the decision-making issue #853: do we use v or vids?

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Explanation for beginners on how this PR was produced:

• igraph_maxdegree() was (re-)enabled in functions-R.yaml
• PARAM_NAMES can be used to rename parameters from the upstream functions.yaml to fit the standards used in the R interface
• In this case, PARAM_ORDER is used not to reorder parameters, but to add the ... parameter to the R interface (denoted by * in the interface definition). In the R interface, parameters following the ... must be spelt out with full name.
• Notice that since ... was added, we need @inheritParams rlang::args_dots_empty in the documentation
• Command to automatically generate the interface: make -f Makefile-cigraph src/rinterface.c R/aaa-auto.R. The two files mentioned in this command are automatically generated. The generated R function (maxdegree_impl) can either be wrapped by the final public function (max_degree()) or in simple cases (like here) just assigned to it.
• We must also run R -q -e 'cpp11::cpp_register()' since a new C function was added.
• We must also run R -q -e 'devtools::document()' to (re-)generate the documentation files in man as well as the NAMESPACE file.
• Please add an example in the documentation block using #' @examples
• Don't forget to add tests for the new function, and run the test suite using devtools::test() before submitting the PR.

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[szhorvat]

Resolved conflicts and rebased on main.