Fixes configclass shared references to keep compound objects independent across subclass instances (#528)

# Description

There is currently an issue with `configclass.py`'s function
`_return_f`, which produces `default_factory` functions for
`configclass` classes.

`configclass` instances which have member variables that are compound
objects, (such as `BaseEnvCfg` and its member `SimulationCfg`) currently
share a reference to a common members (in the example of `BaseEnvCfg`,
its inheriting classes would share a common reference to `self.sim.dt`)

When such classes are inherited, compound objects which have their
member variables changed in the __post_init__ function will be changed
for all subclass instances. I have solved this by changing the non-field
return to be a `deepcopy`

The issue was observed in orbit.eval_sim and was reported this way on
the Institute's JIRA:
> EvalSim will hold the Environment parameters that are set from the
first environment that is loaded in unless it is otherwise explicitly
overridden in the post init or reset EnvCfg member fields by the next
environment.
> 
> Example: 
>
>If EnvironmentCfg A gets loaded first and sets sim.dt = 0.1 followed by
loading EnvironmentCfg B, then Environment B will get the sim.dt set in
Environment A if it does not explicitly set sim.dt in the post_init. Any
fields defined inside BaseEnvCfg that are not redefined inside the Cfg
inheriting BaseEnvCfg will take after the configuration set in the first
environment that gets loaded.

I have added a condition to the test
`test_configclass.test_config_inheritance` which tests this value. Those
wishing to demonstrate that this issue exists can simply change the line
in `configclass.py` back to an assignment and observe the test failing.

## Type of change

<!-- As you go through the list, delete the ones that are not
applicable. -->

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have run all the tests with `./orbit.sh --test` and they pass
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

---------
Signed-off-by: Hunter Hansen <50837800+hhansen-bdai@users.noreply.github.com>
Co-authored-by: Mayank Mittal <12863862+Mayankm96@users.noreply.github.com>