[IMP] purchase(_stock): Add more reporting values

Added new values "Average Days to Purchase" and "Average Receipt Delay" to be
used in purchase enterprise reporting tab where:

  Days to Purchase = confirmation date (i.e. 'date_approved') - creation date
  Receipt Delay: effective Date - ('date_planned' or 'expected_date' if
                 no 'date_planned)

Note that these 2 new values are specific to purchase orders only.
Because of this the standard query result cannot be used since it is
done at the purchase order line level. This means duplication of PO
values occur and results in incorrect aggregation calculations. A hack was
done to ensure that these value will aggregated correctly when used at the
enterprise level. Therefore these 2 new values should only be used in
aggregate.

To support these new measures the following updates were also done:
- Addition/improvement of help descriptions for other date related
  measures to avoid confusion
- Change state string to "Status" to match purchase.order (for
  consistent filtering purposes)

This commit supports "Adapt Reporting" subsection of Purchase KPIs task.

Upgrade PR: odoo/upgrade#877
Enterprise PR: odoo/enterprise#8945
Task: 2198420

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