[ENH] clean-up refactor of TimeSeriesDataSet

[fkiraly]

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

[fnhirwa]

Overall, this clean-up looks good to me 😊

A one point that we should consider:

• I think we can start handling exceptions using if conditions instead of assert as if is optimized for runtime, and for some cases for example if a user is running a python program using optimizer flags(python -O or python -OO) the assert conditions are skipped.

def divide(a, b):
    if b == 0:
        raise ValueError("Denominator cannot be zero.")
    return a / b
print(divide(1, 0) # ValueError: Cannot divide by zero when we do python -O script.py

When we use assert

def divide(a, b):
    assert b != 0, "Denominator cannot be zero."
    return a / b
print(divide(2/0)) # ZeroDivisionError: division by zero with python -O

For the second case, the exception we wanted to handle was skipped and we got an unexpected exception which can affect the exceptions handling in general for the case of our codebase.

[fkiraly]

thanks for review, @fnhirwa - merging so we avoid issues with the linting PR, I will address the assert/raise issue later.