allow table objects to have round_type instead of needing to specify on export

proposals:

```
lyt <- basic_table(round_type = "sas") |>
    ...
```

or 

```
tbl <- build_table(lyt, dat, round_type = "sas")
```

plus a getter and a (recursive) setter.

Different roudnihng types at different parts of the table would be disallowed. Once set the round type should be used automatically by `matrix_form`, `export_as_*` and `toString`.

We (J&J) will prepare a PR.