Remove useless variables from uvp profiles
Add function to bin particle profiles over depth
Improve documentation
New version of read_ids and read_variables which uses the metadata table when available
Rewrite metadata handling

Now read_meta outputs homogenous data.frames wether the metadata table is here or not
Adapt the read_*** functions to the new metadata
Rename add_class to be more explicitly linked to a project
Use stringsAsFactors=FALSE everywhere
Update read_learning_set