Concatenate ensemble output

[dlebauer]

Use PEcAn.base::nc_merge_all_sites_by_year to concatenate all sites and ensemble members from the /out directory into one file / year.

This will make outputs easier to manage and understand.

There are two steps to this task:

• Update ensemble post-processing to generate concatenated files
• Update downscaling to ingest them

Not in scope: concatenating sensitivity runs.

[divine7022]

addressed in ccmmf/downscaling#9