TsDB load from/export to pickle and dataframe

[mariustk]

Summary

• Added support for loading from/exporting to pickle file (using a pandas dataframe format)
  • Implemented to TsDB.load() (strictly, to TsDB._read()) and TsDB.export()
  • Implemented in GUI
• New method TsDB.to_dataframe() generates a pandas dataframe with the common time array as the dataframe index.
• TsDB.export(): new parameter force_common_time enables enforcing a common time array automatically (when possible). Default behavior is not changed (raising exception if not common time array across specified series).
• Minor adjustment to TsDB.create_common_time(): if time array is already common, the time array from either of the time series objects is returned (avoid constructing new array, which may introduce minor inaccuracies/changes to the time array).

Closes #123

[eneelo]

Tasks/checks before approval (@eneelo, @tovop)

• Test new export/import in GUI
• Consider if the test file (data/df_dump.pkl) should be generated during testing initiation (previous experience suggests that pandas dataframes on pickle files may fail to be loaded in a different pandas version than that used to create it).
• Review code

[eneelo]

@mariustk Apologies for delayed response. This looks very promising, but still need to find time to look through the details before approving the pull request. Just wanted to let you know that it isn't forgotten.

[sonarcloud]

Quality Gate passed

Issues
7 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[eneelo]

I have reviewed the code, good job @mariustk 👍
I have also pushed some additional code, including support for export of pickle and to generate a pandas dataframe from a TsDB instance. Will now update the PR header/documentation, then make a new release (hopefully shortly).