Need to keep some global attributes

[agstephens]

Description

Note from ESMValTool team:

One comment on global attributes: Apart from their occasional usefulness for the generation of labels in plots, one group of global attributes seems to me to be quite important and also a frequent source of errors. I am talking about the attributes that clarify the relationship between experiments, particularly, how historical experiments are spawned from the piControl experiments, and how the scenario experiments (sspXXX) continue the historical experiments. In order to allow for correct drift correction, parent_experiment, branch_time_in_parent, branch_time_in_child, etc. must be carried over. If modifications to the time units of the data are necessary, the corresponding changes to these attributes should also not be forgotten.

[agstephens]

We could implement a subset of global attrs to keep per project.
But take no action at the moment.