Writable parsed_value

[soininen]

Parameter definition, parameter value and list value items can now be added and updated using the parsed_value field directly. This removes the need to call to_database() each time before calling these operations.

Implements #479

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 95.74468% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.10%. Comparing base (9570601) to head (56e3f6a).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
spinedb_api/mapped_items.py	95.23%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
+ Coverage   81.05%   81.10%   +0.05%     
==========================================
  Files          76       76              
  Lines        9890     9882       -8     
  Branches     1473     1477       +4     
==========================================
- Hits         8016     8015       -1     
+ Misses       1529     1520       -9     
- Partials      345      347       +2     

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