small changes in multisession palceholder, accessors, and transfer metrics of the file push / pulling system

adding start_datetime to session search outputs ( an utc pd.datetime field instead of a text field )

- documentation on get_values_key_chain
- adding the ability to set "not" as a lookup in the json final nested key
- adding the ability to set "contains" with a list, (and it's case insensitive counterpart), to create a series of backend compatible queries filters, allowing to make sure all "must be contained" values are matched. Works also with __not.
- Note that __not and other lookups positions in the last key are not important, but they must be after the actual json key in any case. Also there must be only one lookup argument like this in a single key. ( plus an optionnal "__not" )

fixing list_dataset typehint on remote class. (deprecating local class, will need to reactivate later)

starting modification of pd acessors for multi-use cases specific to alyx interaction (plan is to have later plots, etc)

adding the registration tool in one directly (rather than registrator wich is in fact more a rule based tool on top of one)