Move BMI variable reference to yaml file(s)

[BSchilperoort]

In #101 I added a dynamic variable reference for the BMI to make it easier to add/modify the STEMMUS_SCOPE variables the BMI can interact with.

Moving this to a yaml file (or multiple yaml files) would make it easier for STEMMUS_SCOPE developers to update the BMI.

see https://github.com/EcoExtreML/STEMMUS_SCOPE_Processing/pull/101/files/d82af5ecc42a85c32fcf6cc9b7a750fec1ba4cc4#r1676011619