Enable general model export in latest ECA version

Problem/Motivation

ECA currently only allows model export if it's being implemented by the modeller. In #3337182: Make all model exportable the model export moves upstream into ECA ModellerBase so that every model can be exported, and the modeller doesn't have to deal with it. This should also be possible with eca_cm. As the ModellerInterface declares the export method, but the base modeller class doesn't implement that to leave it to the modeller itself, eca_cm contains an export method which returns just NULL.

In the next version of ECA, the base modeller comes with a generic export function and the overwrite from the modeller should be removed. As this may cause issues depending on which ECA version is being used, we also need to define a minimum ECA version when releasing this for eca_cm.