Daily liveweight gain

Problem/Motivation

This was a feature in 1.x.

At first we only displayed the daily live weight gain in the animal weight report. This was the dlwg for the average weight in the animal group.

Later we added the ability to view a single animal's dlwg: https://github.com/farmOS/farmOS/pull/309

Some ideas in this issue might make us want to re-think how we compute dlwg: https://github.com/farmOS/farmOS/pull/422

Graphing this on the individual animal's view might be hard since the current 2.x implementation is using a views attachment to graph an individual's weights (I forget if we had this feature previously).

Steps to reproduce

N/A

Proposed resolution

Identify an MVP to implement existing functionality. Likely just the weight report to start.

The individual animal liveweight gain could be implemented in a separate module that adds an additional computed field with this information to the asset. Perhaps a second service as well. Putting it in a separate module makes this an optional feature.

Remaining tasks

Test and implement

User interface changes

Dlwg is calculated and visible in weight report, maybe individual animal page.

API changes

None.

Data model changes

None.