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

@@ -56,11 +56,6 @@ class RollingMixin:
             sampled timeseries, the period number will be casted down to an odd number if
             ``center=True``.
 
-        target :
-            Field to write the result of the rolling calculation to. Will be generated if not existant.
-            If not given, `field` will be overwritten with the calculations result.
-            If multiple fields are given, `target` must be assigned.
-
         func : default mean
             Function to roll with.