[Econ] Process seed costs

[JoeWaddell]

Is your feature request related to a problem? Please describe.
Need additional processing in mapping.py (or preprocessing.py) on Economics branch to handle seed costs.

Describe the solution you'd like
This requires looking at both "field.crop_specification" and "field.field_size" to

A) Convert field size to m2 (to match econ inputs)
B) map many to one crops grown to given commodity_prices

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

Target Deadline
By when should this issue be done.

[allisterakun]

Grooming: @JoeWaddell could you advise on the priority of this issue?

[JoeWaddell]

I'd say fairly high! The SMEs are meeting weekly now to help wrap up the Economics module, so this (and #3080) would go a long way to making that happen. Similar design pattern between the two, and other instances (one more to open on that front, at minimum), so hopefully not a huge lift.