Adjust groupby operations to align with Pandas release 2.1.0

[CBROWN-ONS]

Description

In this PR, groupby operations are updated to their string counterparts. Previously they were passed as functions (functions from the numpy package), e.g., np.mean.
Defences for summaries have also been updated to reflect this change, along with tests.
The error raised by utils::defence.py::_check_list() has been altered slightly to ensure that tests pass when a function is passed.
Relevant docstrings have also been altered.

Fixes #111

Motivation and Context

The motivation for this PR is the FutureWarning generated by Pandas after the release of Pandas 2.1.0. More information on this can be seen in the issue. #111

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Test configuration details:

• OS: Windows 10
• Python version: 3.9.13
• Java version: N/A
• Python management system: Conda

Advice for reviewer

Checklist:

• My code follows the intended structure of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• 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 pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional comments