Changes to ts representation

[jakubchlapek]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Summary

Changed the TimeSeries representation both for terminal and notebook html.

Other Information

The representation no longer relies on xarray, and also should be more intuitive as well as provide a better overview over the data.

[dennisbader]

Very nice, thanks a lot @jakubchlapek this is amazing and makes TimeSeries much more interpretable 🚀

I made the little figure with some suggestions (I hope you can read it :D ). I can also give you the source file if you need it.

The code to reproduce is here:

import pandas as pd
from darts.datasets import AirPassengersDataset
series = AirPassengersDataset().load()
series = series.stack(series / series.all_values()[::-1]).stack(series + 1000.)
series = series.with_static_covariates(
    pd.DataFrame(
        {"sc1": [1,2., 5.], "another_one": ["an example", "yet another example", "this as well"]}
    )
)
series = series.with_columns_renamed(series.columns.tolist(), ["first column", "another column", "third column"])
series = series.with_hierarchy({"another column": ["first column"], "third column": ["first column", "another column"]})
series = series.with_metadata({"file_path": "/this/is/an/example/path.csv", "product id": 145225905, "country": "Switzerland"})
series

Let me know what you think, we can further discuss offline.

[codecov]

Codecov Report

❌ Patch coverage is 12.69841% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.00%. Comparing base (a0cc926) to head (d7fc879).

Files with missing lines	Patch %	Lines
darts/utils/formatting.py	12.24%	43 Missing ⚠️
darts/timeseries.py	14.28%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2931      +/-   ##
==========================================
- Coverage   95.40%   95.00%   -0.40%     
==========================================
  Files         146      147       +1     
  Lines       15707    15768      +61     
==========================================
- Hits        14985    14981       -4     
- Misses        722      787      +65     

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