instead of doing one getInventory by budget cell, we do one getInventoryList
  by budget line.
- Budget variation now uses the variation defined at the proper level, ie if
  this is a line level variation, it uses membership criterion from the line,
  if this is budget level, from the budget. if this is a cell level, from the
  cell. This means that we no longer have to copy all level categories on the
  cell. The UI will have to be updated.
- Test those new features and add some missing tests



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@37220 20353a03-c40f-0410-a6d1-a30d3c3de9de