Add linear and zero-fill interpolation strategies to TimeSeriesDataSe…

[sujanyd]

[ENH] Addition of other filling strategies for missing timesteps in TimeSeriesDataSet (GH-1409)
Reference Issues/PRs
Fixes #1409
What does this implement/fix? Explain your changes.
Added support for multiple filling strategies for missing timesteps in TimeSeriesDataSet when allow_missing_timesteps=True.
Previously, only forward fill was supported. This PR adds:

"forward_fill" — repeats the last known value (default, backward compatible)
"linear" — linearly interpolates between boundary values for real-valued variables, falls back to forward fill for categorical variables
"zero" — fills missing values with 0.0
Dict support — per-variable strategy e.g. {"value": "linear", "cat": "forward_fill"}

Changes made:

Modified _timeseries.py — added interpolation_strategy parameter to init and interpolation logic to getitem
Added docstring for the new parameter
Added test file tests/test_data/test_interpolation.py
Added demo script for visual verification of all strategies

What should a reviewer concentrate their feedback on?

The interpolation logic inside getitem — specifically how gaps are detected and filled
Whether the fallback to forward fill for categorical variables when "linear" is specified is the right behavior
Edge case handling — gaps at sequence boundaries, multiple consecutive gaps
Backward compatibility — default behavior should be identical to before this change

Did you add any tests for the change?
Yes. Added tests/test_data/test_interpolation.py which covers:

Forward fill (default behavior preserved)
Linear interpolation across a gap
Zero fill
Mixed per-variable strategy using a dictionary

Any other comments?

A demo script is included to visually compare all three strategies on a sample dataset with a gap
Linear interpolation is only applied to real-valued variables — categorical variables always fall back to forward fill regardless of strategy specified
This is a purely additive change — no existing behavior is modified

PR checklist

The PR title starts with [ENH]
Added/modified tests
Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
To run hooks independent of commit, execute pre-commit run --all-files

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