Add ui for specifying the study in the DATA ENTRY section of the user profile:
The preferences will be stored in XWiki.ConfigurationClass objects in the user profile page (see also Technical information for PT-3780).
XWiki.ConfigurationClass has two free-text (String) fields: property and value. The study preference will be stored as follows:
Property=defaultStudy / value = <a study reference>. Example: defaultStudy / xwiki:Studies:TheGreatStudy.