Add deep_copy_value() function

[soininen]

We need a function that make deep copies of parameter values.

Re spine-tools/Spine-Toolbox#2657

Checklist before merging

• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 78.26%. Comparing base (5840d4b) to head (56a6a78).

Files	Patch %	Lines
spinedb_api/parameter_value.py	92.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #377 +/- ## =========================================== + Coverage 78.22% 78.26% +0.03% =========================================== Files 75 75 Lines 9435 9460 +25 Branches 1999 2009 +10 =========================================== + Hits 7381 7404 +23 - Misses 1724 1725 +1 - Partials 330 331 +1 ```

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