Set default in-memory value depending on the dataset size
Set a default value for `in_memory` depending on the size of the dataset to be loaded.

Close #2179.

TODO:
- [x] Add a section in the docs about this.
- ~Add a warning if someone tries to specify `cache_file_name=` in `map`, `filter` etc. on a dataset that is in memory, since the computation is not going to be cached in this case.~