cachedataset is a misleading name for the function

Many users have construed that the `cachedataset()` function (of the `SIARecord` and `SSARecord` classes) involves the use of a special storage area on disk that can avoid having to re-download a file multiple times in a session. Instead, this function simply saves the dataset to a specified file. We should consider changing the name to something like `save_dataset()`. This will become more important as we integrate more with astropy and take advantage of its caching capabilities.