estimatedDaysOfStock: a server-calculated field processed for inventory item adjustments

[tangollama]

One of the objectives of the inventory system is to help hospital admins know what items are and are not in danger of stocking out. To support this, we added a rank field (A,B,C) through the following issues for frontend:

HospitalRun/hospitalrun-frontend#207
HospitalRun/hospitalrun-frontend#209

Related to this PR (HospitalRun/hospitalrun-frontend#232), we now need a new calculated field on an item that is recalculated each time stock is adjusted that estimates the number of days left of stock.

Essentially, we need to run the Days Supply Left In Stock report for a given item for the last year of activity and then stores that value in a field called estimatedDaysOfStock. That will allow the stock "condition" calculated field to accurately represent both the relative importance and quantity of a stock item.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.