Add formula parser support for average calculation

[shsms]

What's needed?

#137 adds support for calculating averages using the FormulaEngine. But formulas that calculate averages can currently only be built from code.

Creating such formulas should be possible through the formula parser as well.

Proposed solution

No response

Use cases

This would allow us to override the SoC formula from the UI, or from a config file, for example.

Alternatives and workarounds

No response

Additional context

No response

[christianparpart]

Creating such formulas should be possible through the formula parser as well.

Can we add to the requirement what the anticipated syntax should ideally look like?