[FIX] project_timesheet: date of created analytic line not in user tz

When the user was in timezone UTC + 1, and added a project.task.work which created an analytic entry line (timsheet activity), if the datetime was set to 11/25 00:00:00, the date on the analytic line was set to 11/24, because this was the truncated stored value(UTC Time), which was 11/24 23:00:00
[FIX] stock: compute average price on action_done

the average price computation is now deduped in a separate function
called in stock_move action_done.
[ADD] purchase: average price computation test