create a dm_explicit_na for preprocessing.

[BFalquet]

Create a function that converts all character (but also logical and factor) columns of a dm object into factor with explicit NA levels, for instance <Missing>. The goal is to speed up data pre-processing instead of attending separately each column that has to be converted to factor with explicit missing values (which is often required by the rtable functions).

Requirements:

• give the option to exclude some tables
• give the option to exclude some columns
• give the option to omit character columns
• give the option to omit factor columns (by default, logical should not be transformed)
• give the option to choose the missing level representation (by default <Missing>)