Adapt books for analyses not stratified by visit (e.g. SChISM)

- Fix minor errors
  - Replace groups name in  some titles by their associated label
- Fix statistical errors
  - When there were NAs in features, account for NA as strate -> pb to determine the real sample size in independence tests for continuous features (`p_value_continuous`)
  - na.omit before regression...
  - ... that could induce the appearance of zero-variance features. These ones have to be remove or it can lead to error
- Fix problems linked to VISIT = TRUE
  - Group results were not displayed
  - Did not display the table of count of features per group because it needed a column VISIT that does not exist in this case
- Aesthetics
  - Replace NAs in tables by white space

See merge request compo/compoeda!63

Adapt books for analyses not stratified by visit (e.g. SChISM)

See merge request compo/compoeda!63

Adapt books for analyses not stratified by visit (e.g. SChISM)

See merge request compo/compoeda!63