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/funcs/interpolation.py

@@ -144,7 +144,6 @@ class InterpolationMixin:
         method: _SUPPORTED_METHODS,
         order: int = 2,
         limit: int = 2,
-        downgrade: bool = False,
         flag: float = UNFLAGGED,
         **kwargs,
     ) -> "SaQC":
@@ -174,11 +173,6 @@ class InterpolationMixin:
         flag : float or None, default UNFLAGGED
             Flag that is set for interpolated values. If ``None``, no flags are set at all.
 
-        downgrade : bool, default False
-            If `True` and the interpolation can not be performed at current order, retry with a lower order.
-            This can happen, because the chosen ``method`` does not support the passed ``order``, or
-            simply because not enough values are present in a interval.
-
         Returns
         -------
         saqc.SaQC
@@ -187,8 +181,7 @@ class InterpolationMixin:
             self._data[field],
             method,
             order=order,
-            inter_limit=limit,
-            downgrade_interpolation=downgrade,
+            limit=limit,
         )
 
         interpolated = self._data[field].isna() & inter_data.notna()
@@ -211,7 +204,6 @@ class InterpolationMixin:
         method: _SUPPORTED_METHODS,
         order: int = 2,
         limit: int = 2,
-        downgrade: bool = False,
         **kwargs,
     ) -> "SaQC":
         """
@@ -241,12 +233,6 @@ class InterpolationMixin:
             Maximum number of consecutive 'nan' values allowed for a gap to be interpolated. This really restricts the
             interpolation to chunks, containing not more than `limit` successive nan entries.
 
-        downgrade : bool, default False
-            If `True` and the interpolation can not be performed at current order, retry with a lower order.
-            This can happen, because the chosen ``method`` does not support the passed ``order``, or
-            simply because not enough values are present in a interval.
-
-
         Returns
         -------
         saqc.SaQC
@@ -283,8 +269,7 @@ class InterpolationMixin:
             data=datcol,
             method=method,
             order=order,
-            inter_limit=limit,
-            downgrade_interpolation=downgrade,
+            limit=limit,
         )
 
         # override falsely interpolated values:
@@ -307,7 +292,6 @@ class InterpolationMixin:
                 "method": method,
                 "order": order,
                 "limit": limit,
-                "downgrade": downgrade,
                 **kwargs,
             },
         }