Chemical Mixhaps

### Testmerges

#1834

### Description of the bug

When combining chemicals, especially as an OT, occasionally some error occurs and there will be some crazy decimal value left as extra. Not usually a big deal, but it can be annoying to remove the extra. When mixing ANFO earlier as an OT, I filled a bucket with welding fuel and manually added ammonia nitrate below the threshold to explode. During one of these mixtures, I believe a trace amount of ammonia nitrate was left over, so when I added an extra 60u of ammonia nitrate to the whole container of fuel, that trace amount also was included in the reaction and caused all of OT to be destroyed.

### What's the difference with what should have happened?

That .22222211231u of ammonia nitrate shouldn't have been left over.

### How do we reproduce this bug?

1. Play as an OT. When combining chemicals, this usually happens,

### Issue Bingo

- [X] Issue could be reproduced at least once
- [X] Issue happened in a recent (less than 7 days ago) round
- [X] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)