Several fixes and refactorings to the interpolation methods

Fixes #388 (closed), unfortunately the implementation of interpolateNANs with limit is not None is quite a bit slower, then the incorrect previous one. Any ideas to speed things up are highly appreciated, I couldn't find a better solution despite a few hours of trying.

tests/lib/test_ts_operators.py

 # SPDX-FileCopyrightText: 2021 Helmholtz-Zentrum für Umweltforschung GmbH - UFZ
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
+from __future__ import annotations
+
 import numpy as np
 import pandas as pd
 import pytest
-from numpy.testing import assert_array_equal, assert_equal
 from pandas.testing import assert_series_equal
 
 import saqc.lib.ts_operators as tsops