gnosis / dex-services / 3707

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travis-pro

Pricegraph underflow (#1012)

Fixes #916 

This PR adjusts the rounding margin to be based on the amounts being considered.

### Test Plan

Fixed fuzzer test, 
```
$ git checkout c792d0c
$ cargo test -p pricegraph
underflow panic!
$ git checkout pricegraph-underflow
$ cargo test -p pricegraph
ok!
```

### Commit History

* create failing unit test

* implement fix

* assert amount

* assert amounts

* better rounding error calculation

* fix constant name

* more documentation

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