Sample by default the same number of rows as in the original table

[csala]

Current version samples 5 rows by default when no explicit number is passed.

Let's store the number of rows that were passed in the original tables and sample that number by default.

Note that, if sample is used, the default number of sampled rows will be the same as in the corresponding original table, but if sample_all is used, only the parent tables will have the same number of rows, as the number of rows in the children table will also be sampled.