Implement "Sales" when a resource is native to that planet

For instance, you are on planet XYZ and they have resource ABC as a native resource - so when you are there it's 50% the cost.

Right now I'm just using "cost" for the inventory and math... so I'm going to need to write like a "buyWithSale" or something which will take in a saleAmount then handle all the math that way so the graphs will remain correct (I don't want to mess with the actual cost - or perhaps a "buyStock(salePrice)" function which will take an argument that could be a sale price?)
## Dependencies
- [ ] Issue https://github.com/evangipson/microstocks/issues/34
- [ ] Issue https://github.com/evangipson/microstocks/issues/39

No files, tests, or entry points are named. First review dependent issues 34 and 39, then trace the inventory cost, buying, and graph calculations; the work is done when native resources cost 50% on their planet without changing the recorded cost or breaking graph values.

Written by the indexing model from the issue text.