[ENH] clean-up refactor of TimeSeriesDataSet (#1746)

This PR carries out a clean-up refactor of `TimeSeriesDataSet`. No
changes are made to the logic.

This is in preparation for major work items impacting the logic, e.g.,
removal of the `pandas` coupling (see
#1685), or a 2.0
rework (see #1736).
In general, a clean state would make these easier.

Work carried out:

* clear, and complete docstrings, in numpydoc format
* separating logic, e.g., for parameter checking, data formatting,
default handling
* reducing cognitive complexity and max indentations, addressing "code
smells"
* linting

Author: fkiraly