* Add fill group for filling
* Fill group check if there is a missing date too
* Add period argument to function, to check for missing dates

Add scenario with missing date
Adjust test function to use period column

In order to check if strata, reference difference is not 0 we need to
sort the values.

* Fill group works only with diff(1) since we are seperating the values
by strata and if missing date exists.
* astype(int) is redundant, when summing True/False same as 1/0
* Add kwargs and update docstrings