Benchmarks: simplify models

Context

Models declared for benchmarks need to specify their parameter names.

For instance, when declaring that a benchmark has an affine temporal complexity, we need to give a name to the intercept and coefficient values, as is done here.

However, these names are often the same: benchmark_name ^ "_const" and benchmark_name ^ "_coeff", but not always since the convention is not made explicit somewhere.

This request allows to place the responsibility to create the names to the system, which formally explicits the convention with default names.
It will bring some uniformity to the names, making them more readable, simplify model declaration by not having to provide names, while still keeping the possibility for the user to define her own names.

The new models are used on the skip list benchmarks.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR