Adding "environmental process association" class

[timalamenciak]

Is your feature request related to a problem? Please describe.
I want to modify Biolink Model to include an association that describes the relationship between environmental variables and environmental processes. This will be very helpful in the field of ecology for annotating research on ecological processes and variables.

Describe the solution you'd like
I would like to add the association as well as some helper enums, all part of the subset "ecolink"

What working group (or team) did this request originate from?
None

Additional context
There is a paper in draft describing the usage of the Ecolink Model in more detail. I will also make a PR to address this issue and code review is most welcome.