Quantity unit conversions

[arcnmx]

This is just a rough attempt at using the amount and quantity configured in grocy's product barcode for purchase and consume amounts. This likely relates to #209/#161/#123. Adding a config toggle that uses the default consume/purchase quantity+amount instead of the barcode's would probably make sense to address #215 as well.

[jtitley]

@Forceu bump on this.

[Forceu]

Hi, thanks for the PR. This is still set as draft, is it ready for review?

[arcnmx]

It should at least be ready for a preliminary review - most of the reason that it's marked WIP is because I'm looking for some initial feedback on whether the approach is worth polishing up for submission.
The code itself is basically ready as-is (besides the potential inclusion of settings to customize the behaviours mentioned previously), it mostly just needs some polish and clean-up in the form of formatting and more documentation before I'd consider it "ready".

[gstburnside]

@arcnmx this will be a great help if I'm following correctly. I am currently creating a number of barcodes on a given product to add/consume "x" quantity units, but scanning them is always adding/consuming the product stock unit instead.