RollingRMSE: Extraction time index from the wrong data

[benHeid]

Describe the bug
In Line 79 we extract the time index from kwargs and use this time index on y. This leads to an error if the name of the time index in kwargs and y differs.

Similiar issue in line 85 and 86 where we set the name of the time_index to time. However, in general that would not match the name of the time dimension of the result or of y.

Expected behavior
In all three cases _get_time_indeces(y)[0] is used.