A table for harvests

[mhauru]

Add a new table under Crops that lists harvests only.

[mhauru]

A basic version of this now exists. Items remaining:

• [ ] Add more variables: Yield per area, more percentiles for T&RH, irrigation, column, shelf. At a later stage maybe also splitting the grow period in to early, mid etc. (separate issue for that?)
• [x] Turn the raw SQL query into SQLAlchemy
• [ ] Make an API end point? Or at least unify things so that batch_list, batch_details, and future visualisations can easily use the same fancy query stuff.
• [ ] Make sure that the time range picks batches based on harvest time. This should be applied elsewhere too as the rule, where it makes sense (batch_list doesn't always have a harvest time)