chore: Fix Pandas deprecation warnings for future compatibility

[noamjgal]

Description

When using Trackintel with the latest Pandas versions, numerous FutureWarnings are displayed concerning inplace operations and downcasting. These warnings appear in specific locations and will lead to actual errors in future Pandas versions.

Warnings observed:

1. Chained Assignment with Inplace Operations:

   FutureWarning: A value is trying to be set on a copy of a DataFrame or Series through chained assignment using an inplace method.
   The behavior will change in pandas 3.0. This inplace method will never work because the intermediate object on which we are setting values always behaves as a copy.
   

2. Downcasting Deprecation:

   FutureWarning: Downcasting object dtype arrays on .fillna, .ffill, .bfill is deprecated and will change in a future version. Call result.infer_objects(copy=False) instead. To opt-in to the future behavior, set `pd.set_option('future.no_silent_downcasting', True)`
   

These warnings appear in these specific locations:

1. positionfixes.py:361: pfs["tripleg_id"] = pfs["tripleg_id"].ffill()
2. triplegs.py:274: sp_tpls["is_activity"].fillna(False, inplace=True)
3. triplegs.py:93: sp_tpls["temp_trip_id"].ffill(inplace=True)
4. triplegs.py:157: trips_with_act["is_activity"].fillna(False, inplace=True)

Environment

- Python version: 3.11.8
- Pandas version: 2.2.3
- Trackintel version: 1.3.1

Proposed solution

According to Pandas documentation, the recommended solution is to:

1. Replace df[col].method(value, inplace=True) with df.loc[:, col] = df[col].method(value)
2. Add downcast=None to ffill/fillna operations to prevent silent downcasting

Steps to reproduce

The warnings can be reproduced by running any code that calls the generate_staypoints, generate_triplegs, or generate_trips functions from Trackintel with any dataset.

[hongyeehh]

Thanks for the warning report! These will be worked on in a later version of Trackintel.

We also welcome a new PR to integrate these future compatibility.

[bifbof]

@noamjgal
These warnings are already fixed on the master branch if you don't want to wait for the next release. :)
See #623

[hongyeehh]

The warnings were fixed with the latest release. Enjoy trackintel :)