Move enkf_node implementation of FIELD

[oyvindeide]

We have been doing refactoring of the callbacks and sampling, and should be ready to save data differently from before. This includes removing enkf_node implementation FIELD which is loading parameters from the forward model in the OK callback or in the creation of the run path. We should try to drop field.cpp and instead load the field using XTGeo, and probably just save the field as is.