Properties of `SatelliteDataset` should be inferred from dataset by default

Things like `sample_freq_mins` can be inferred from any given dataset, e.g. by calculating the modal diff. I ran into this when forgetting what frequency I downloaded my dataset with!

Locate SatelliteDataset and inspect how sample_freq_mins and other properties are currently set. Try the inference on a dataset with a known sampling interval, using the modal difference between observations, and consider the work done when properties are inferred by default without requiring the downloader to remember the frequency.

Written by the indexing model from the issue text.