Dev: Clean up tab in frontend_helper
Dev: add some PHP doc comment
Dev: remove deprecated function : checkconfield, checkmandatorys, checkconditionalmandatorys
Dev: remove deprecated function : remove_nulls_from_array
New feature #8040: STATA export functionality

DEV: Implements XML-export for statistical software STATA
DEV: Thanks to mdekker for his support regarding the plugin
DEV: architecture.