Add Expected energy as nutrient

[aleene]

Description

The expected energy is the energy expected based on the values for the nutrients and a standard formula. This expected energy should be the same as the published energy. Any large deviations indicate a problematic product. In a scatter plot these are easily seen and can be inspected and corrected.
Large deviations in the energy difference (publish energy minus expected energy) can be used as a flag for data quality.

Acceptance criteria

Some test to check the correctness of the formula based on a specific complex product.

What would a demo look like

A scatter plot with published energy versus expected energy for a category.

Notes

• See the wiki;
• Exclude upper values from calculations;

Tasks

• Add the expected nutrient to nutrients taxonomy;
• Implement the formula and calculations for each product;
• Add the expected energy to the statistics and graph module;

[aleene]

This is availabe through the Power user script, which is enough for the moment.