added check for untrained models for copying of model parameters

[dennisbader]

Addresses #724.

Summary

• instantiating ensemble models with fitted/trained forecasting model resulted in deepcopy error.
• added check for untrained models with explanatory error message.

[codecov-commenter]

Codecov Report

Merging #728 (93d2a39) into master (8fa7dbf) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #728   +/-   ##
=======================================
  Coverage   90.81%   90.81%           
=======================================
  Files          67       67           
  Lines        6731     6731           
=======================================
  Hits         6113     6113           
  Misses        618      618           

Impacted Files	Coverage Δ
darts/models/forecasting/ensemble_model.py	93.87% <100.00%> (+0.12%)	⬆️
darts/utils/likelihood_models.py	96.03% <0.00%> (-0.01%)	⬇️

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[hrzn]

Thanks!