Applies TotalInvTracker and (R,Q)/(s,S) inventory policies to storage

[nuclearkatie]

Relies on Cyclus PR cyclus#1646. Until that one is merged, this one cannot be merged. They should be merged together if possible

This PR implements the new TotalInvTracker from cyclus#1646 into Storage, which provides an agent-wide inventory limitation and is required as part of initializing the buy policy.

This fixes the janky way that Storage was previously calculating an agent-wide limitation! Now, the max_inv_size is applied to the whole facility through the use of TotalInvTracker. Closes #554, Storage no longer needs to change the capacity of Resource Buffers ever time step

This also adds two variables

• reorder_point when total facility inventory is above this point, no new demands will be made. At or below this point, the agent will request material
• reorder_quantity when applicable, agent will request exactly this amount. It will make an exclusive demand for exactly this value

When reorder_point is used alone, it results in a (s,S) or min/max inventory policy. When the buffer inventory is below reorder_point, buypol tries to buy up to a full facility (max_inv_size, which is facility-wide). When used with reorder_quantity, this is a (R,Q) buy pol where the facility makes a single exclusive request of size reorder_quantity once below reorder_point. In both cases, when the inventory is above reorder_point, no requests are made

Closes #577, closes #578

[gonuke]

This looks pretty clean - looking forward to reviewing it with a working kernel.

[gonuke]

Thanks @nuclearkatie - nice to see this implemented