Provide an API service to calculate areas and lengths

Problem/Motivation

It would be useful for farmOS projects to have a common and reliable way to calculate areas and lengths. It could have usage in core as the issues provided by @m.stenta

Proposed resolution

- An additional service that is based on the calculation functions provided in 1.x https://github.com/farmOS/farmOS/blob/7.x-1.x/modules/farm/farm_map/farm...
- A number of high level methods that allow the calculations required per entity (asset/log) that were available as functions (with a different data model) in farm_area in 1.x: https://github.com/farmOS/farmOS/blob/c5c15043cf30275fa1bdadf8e39a9393ff...

Data model changes

Not yet, but as @paul121 has explored, we could have additional fields on the assets/logs that are calculated upon save? https://github.com/Vital-Agronomics/farm_asset_area or a computed field. TBD.