[FIX] removes non stored fields from measures

Non stored functional fields cannot be grouped by read_group, so it is
useless, and even harmful, to display them in the measure list (in
graph view).
Merge pull request #521 from odoo-dev/master-graphview-measures-fix-ged

[FIX] graph/pivot tables: remove non stored fields from available measures
Merge branch 'master' of https://github.com/odoo/odoo into master-win32