WIP: Inventory api: interpolation + group by time sequence

!6 rebased and in a big
commit so that we can start using that
SimulationTool: implement "linear" flow API

Conflicts:
	product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/Resource_zGetInventoryList.xml

testInventoryAPI: silent pyflakes warnings

more interpolation implementation

inventory_list interpolation flow: support at_date & to_date

more and more interpolation API

interpolation

interpolation

group_by_time_sequence_list

testInventoryAPI: move inventory valuation methods in a dedicated test class

This should not be in test inventory list.

test: move Tracking API to a dedicated test module

inventory api: dirty way of disabling cache if keys such as node_category are used.

interpolation: security on SimulationTool_zGetInterpolationMethod