Horizontal axis rolling

Although a general axis keyword as proposed in #295 (closed) might not be a good idea, - saqc has a functional gap when trying to apply function on rows instead of columns (for example counting the meassurements present at a certain timestamp or calculating the scattering of meassurements at a certain timestamp.)

So i added that "row wise" functionality to qc.rolling.

The reason to add it to rolling, is, that it seems probable, that one might not only want to check for a statistics or an aggregation result at an exact timestamp, but within a certain extension. That can be achieved with a 2 dimensional rolling window.

For example: Get the number of values present within every hour:

qc.rolling(['data1','data2','data3'], func='count', window=1h, target='count')

saqc/funcs/rolling.py

@@ -81,7 +81,7 @@ class RollingMixin:
                 **kwargs,
             )
         else:
-            if not kwargs.get("target", False):
+            if not "target" in kwargs:
                 raise ValueError(
                     "Target has to be assigned for cross statistics calculations."
                 )