Implement ensemble-member support

[spencerahill]

Only None and 0 currently work as ens_mem. Should be able to input integer in range 0 to N-1 for an an ensemble of N members. If no ensemble members, only None should be accepted. Should also be able to input 'mean' to compute and save the ensemble mean.

And, for runs with multiple ensemble members, the 0 input is being ignored. No ensemble member tag being used in writing the data to disk, so there's not even any indication that the data is a single ensemble member, and if we got multiple working without fixing this part, they would just overwrite one another's data.

All of this worked once upon a time, but looking at the code base its mostly gone. So need to re-implement. Should be pretty straightforward,

[spencerahill]

Since #248, this is no longer a bug and instead a feature request.