metrics by session_id with NA values in column

[LoannPeurey]

Metrics pipeline fails when NA values in the session_id column, it should throw a meaningful error (currently just 'Index out of bounds')

[lucasgautheron]

I don't think it's a bug, as long as the package throws a meaningful exception (e.g. "grouping column {column} has nan values")
But then it should be easy to filter out (manually) the recordings without any session_id.