Implementation of the Mehlman et al. benchmark into the full model infrastructure

Fixes #512

Task List

[x] Defined the tests that specify a complete and functioning change (It may help to create a design specification & test specification)
[x] Implemented the source code change that satisfies the tests
[x] Documented the feature by providing worked example
[x] Updated the README or other documentation
[x] Completed the pre-Request checklist below

Change Description

Implementation of the dynamics benchmark test case from Mehlmann et al. (2021). This involves

Creating new initial conditions via a python script
Small fixes to the existing "benchmark" forcing classes

Test Description

Run the model using the config_benchmark.cfg config file and visually compare the output with previous results.

A proper integration test is covered by #524

Documentation Impact

Documentation is covered by #520

Other Details

The "benchmark coordinates" now assume that the box is 512 x 512 km (as in the paper). This means that using AtmosphereBoundaryModule = Nextsim::BenchmarkAtmosphere with a different grid (e.g., one of the realistic ones) may yield unexpected results.

Pre-Request Checklist

[x] The requirements of this pull request are fully captured in an issue or design specification and are linked and summarised in the description of this PR
[x] No new warnings are generated
[x] The documentation has been updated (or an issue has been created to track the corresponding change)
[x] Methods and Tests are commented such that they can be understood without having to obtain additional context
[x] This PR/Issue is labelled as a bug/feature/enhancement/breaking change
[x] File dates have been updated to reflect modification date
[x] This change conforms to the conventions described in the README

nextsimhub / nextsimdg

Issue512 benchmark2 #525