Refactor time series model API to use xarray coordinates

Removed the _convert_timeseries_data helper and refactored BayesianBasisExpansionTimeSeries and StateSpaceTimeSeries to extract datetime and exogenous variable information directly from xarray DataArray coordinates. This improves API consistency, reduces reliance on external coords dicts, and ensures proper validation of input data. Updated test to use datetime values in obs_ind coordinate for compatibility with the new API.

Author: drbenvincent