Take inventory into account when generating purchase order

This functionality is shipping in 1.2.6. Not all unit conversion scenarios are supported at this time so the option to take the inventory into account is not enabled by default and needs to be turned on at the final step of the purchase order process.
Currently supported conversion:
- Mass to mass
- Volume to volume
- Mass to volume, if density is specified
- Volume to mass, if density is specified
- Abstract to abstract, if abstract units are identical
- Inventory mass to abstract, if abstract unit has mass specified
- Inventory volume to abstract, if abstract unit has volume specified
Counting inventory using abstract units and then using that inventory count to negate the required purchase order amount that is specified in mass or volume is currently not supported. We will build this in the next iteration of this feature.