Fix: Improve TimeSeries.getitem frequency inference #4555

Job	Run time
lint	43s
tests (macos-latest, 3.9, all)	40m 42s
check-examples (03-FFT-examples.ipynb)	3m 27s
tests (ubuntu-latest, 3.9, all)	31m 27s
docs	9m 26s
check-examples (04-RNN-examples.ipynb)	7m 48s
check-examples (00-quickstart.ipynb)	6m 33s
check-examples (02-data-processing.ipynb)	6m 18s
check-examples (01-multi-time-series-and-covariates.ipynb)	7m 35s
	1h 53m 59s

Provide feedback