The possibility to add growing season is missing

[HenriKajasilta]

Description:
Farmers should have the option to enter the growing season somewhere. This should be given as a time period (e.g. d/m/y - d/m/y).

Proposed solution:
At the moment, it is not yet clear under which activity this should be placed, but this is perhaps related to the observations.

Notes:
In the Field Observatory there is a default growing season, but in reality that might not hold and it would be better to allow farmers to add it on their own.