Improved implementation for pars_dict and pars attributes + pickle test

[taldcroft]

Description

This improves on the fix from #142 using cached_property to define the pars_dict and pars properties with defaults as {} and [] respectively.

It also fixes poor coding style for a couple of properties.

This is not needed for Ska3 2025.1, it can go in a later release.

Interface impacts

Testing

Unit tests

• Mac

(ska3-flight-2025.0rc2) ➜  xija git:(recursion-redux) git rev-parse --short HEAD
39633c7
(ska3-flight-2025.0rc2) ➜  xija git:(recursion-redux) pytest xija
===================================================== test session starts ======================================================
platform darwin -- Python 3.12.8, pytest-8.3.4, pluggy-1.5.0
rootdir: /Volumes/git
configfile: pytest.ini
plugins: doctestplus-1.3.0, anyio-4.7.0, timeout-2.3.1
collected 44 items                                                                                                             

xija/tests/test_get_model_spec.py .......                                                                                [ 15%]
xija/tests/test_models.py .....................................                                                          [100%]

===================================================== 44 passed in 34.85s ======================================================

Independent check of unit tests by [REVIEWER NAME]

• [PLATFORM]:

Functional tests

(ska3-flight-2025.0rc2) ➜  xija git:(recursion-redux) python -m xija.gui_fit.app acisfp_spec_matlab --stop 2025:001

This worked as expected, in particular fitting succeeded.