First implementation of a generic _activate_form method

Not complete yet.
Use _activate_form() in login() method.
Clean-up login() method to call authenticate only once.
Simplified query_instrument() using _activate_form().
Merged authenticate() into login()
Fixed missing '/'
Improved handling of file upload with forms.
Made not implemented enctype-dependent post more verbose.
Implemented handling of 'application/x-www-form-urlencoded'
Converted form_dict from dictionary to list. Added handling for CheckboGroup.
Initial implementation of a direct data_retrieval() method.

For now, it only returns the response object of the data download page.
Work in progress…