bug: jsonpickle is needed by some submodules but not en explicit dependency of the package

[jmp75]

Reported by James B.

This is deliberate, but a stumbling block for new or even existing users. Reconsider.

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Cell In[1], line 13
     10 # SWIFT-specific imports
     11 from typing import TYPE_CHECKING, Optional
---> 13 import swift2.doc_helper as std
     15 from uchronia.time_series import mk_date
     16 from swift2.classes import CompositeParameteriser, Simulation

File \\path\toCode\Python\.venv\Lib\site-packages\swift2\doc_helper.py:5
      2 import os
      3 from typing import Dict, Iterable, List, Optional
----> 5 import jsonpickle
      6 import numpy as np
      7 from cinterop.timeseries import end_ts, start_ts

ModuleNotFoundError: No module named 'jsonpickle'