Add starting_ensemble_id namelist option

[abrooks1085]

Description
This PR adds a new namelist variable, starting_ensemble_id, to ensemble_manager to support split ensemble experiments. This change introduces starting_ensemble_id (default = 1). When ensemble_size > 1, the filename appendix is now computed as

global_ens = starting_ensemble_id + ensemble_id - 1

Fixes #1818

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[uramirez8707]

This just needs documentation, but otherwise it looks good to me.

[uramirez8707]

Let's document this new namelist