TimeSeries and EventSeries window function should be able to window based on non-default frequencies

[cassandralem-msu]

For example with the student data, window based on semester indices (i.e. 2 semesters)

[mnky9800n]

i think this would require creating a custom index so that it overlaps with other data that also has a comparable custom index

[mnky9800n]

it should be called something like RelativeTimeIndex and could have a starttime but then the increment is an integer