Load only first part of table for table preview

[hagenw]

In #97 we introduce a table preview, that relies on downloading the whole table into cache.
We might update the behavior later to read only the first part of a table (which is possible with parquet and csv tables).

The easiest solution might be to first add this feature to audformat, and also provide a way in audb to download a table without writing a pickle file to cache.