Duplicate DBModel/Script prevention

[amandarichardsonn]

This PR prevents duplicate ML models and scripts names being added to an Ensemble member if the names exists already.
The checks are performed for Ensemble.add_ml_model(), Ensemble.add_model(), Ensemble.add_script() and Ensemble.add_function().

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (18fe50a) 90.55% compared to head (4bcdc4d) 90.74%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #475      +/-   ##
===========================================
+ Coverage    90.55%   90.74%   +0.19%     
===========================================
  Files           60       60              
  Lines         3821     3826       +5     
===========================================
+ Hits          3460     3472      +12     
+ Misses         361      354       -7     

Files	Coverage Δ
smartsim/entity/ensemble.py	99.25% <100.00%> (+0.04%)	⬆️

... and 2 files with indirect coverage changes

[MattToast]

Couple of small change requests but nothing worth holding up approval over! LGTM!!