Make RecData.format_amount_string_from_amount a staticmethod

[cydanil]

Description

closes #314

How Has This Been Tested?

This was tested by adding ingredients manually.
It turns out, it's hard to test

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.