initial small changes to add returns_id attribute to APOSMM class

[jlnav]

I'm assuming that if-and-only-if generators return IDs that we'll want to map those to sim_ids. But what other conditions do we have to account for?

What behavior changes does libE still need, depending on if a gen returns IDs or not?

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.37%. Comparing base (ea9c35c) to head (d12f53d).
⚠️ Report is 2 commits behind head on experimental/jlnav_plus_shuds_asktell.

Files with missing lines	Patch %	Lines
libensemble/generators.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                            Coverage Diff                            @@
##           experimental/jlnav_plus_shuds_asktell    #1646      +/-   ##
=========================================================================
- Coverage                                  58.37%   58.37%   -0.01%     
=========================================================================
  Files                                         80       80              
  Lines                                       8125     8127       +2     
  Branches                                    1234     1235       +1     
=========================================================================
+ Hits                                        4743     4744       +1     
  Misses                                      3099     3099              
- Partials                                     283      284       +1     

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