Add max length of column

[Pierlou]

[Maybe not : CHARACTER VARYING instead]
Useful for SQL uploads later on, to set the VARCHAR max length (ould use TEXT but impossible to set an index on a TEXT column)
Could be in the profile section :
{'tops': [
{'count': 2772, 'value': 'TTTTTTTT'},
{'count': 780, 'value': 'XXXXXXXXXX'}}],
'nb_distinct': 10,
'nb_missing_values': 28,
'max_length': 12}