Introduce add_update and use it in import_data

[manuelma]

Fixes #305

Checklist before merging

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

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (dd0d4dc) 77.21% compared to head (90caffe) 77.09%.
Report is 4 commits behind head on 0.8-dev.

Files	Patch %	Lines
spinedb_api/db_mapping.py	82.50%	7 Missing ⚠️
spinedb_api/db_mapping_base.py	71.42%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           0.8-dev     #320      +/-   ##
===========================================
- Coverage    77.21%   77.09%   -0.13%     
===========================================
  Files           74       74              
  Lines         9244     9208      -36     
  Branches      1964     1947      -17     
===========================================
- Hits          7138     7099      -39     
- Misses        1773     1777       +4     
+ Partials       333      332       -1     

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