relabel columns which has been reshaped (spread) by factor level

This code in `merge_datasets` relabel only columns which are not spread (get_relabel_cols).

```
anl_relabel_call <- get_anl_relabel_call(
columns_source = get_relabel_cols(columns_source, dplyr_call_data), # don't relabel reshaped cols
datasets = datasets,
anl_name = anl_name
)
```

For example AVAL_OS and AVAL_PFS should not be relabelled by column label but by factor level of variable which spreads data (eg PARAM). Optimally if possible should be in one rtables::relabel call but sepatate call also allowed.

Provenance:
```
Creator: gogonzo
```

Start in merge_datasets and trace get_relabel_cols and get_anl_relabel_call to understand how reshaped columns are excluded. Reproduce a spread by PARAM involving columns such as AVAL_OS and AVAL_PFS, then verify that spread columns use factor-level labels while other columns retain column-label relabeling, using rtables::relabel where appropriate.

Written by the indexing model from the issue text.