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.

saqc/lib/ts_operators.py

@@ -9,6 +9,8 @@
 """
 The module gathers all kinds of timeseries tranformations.
 """
+from __future__ import annotations
+
 import re
 import sys
 from typing import Literal, Union