Added predict_item to RepresentablePredictor and adapted subclasses.

[jaheba]

Addresses #238.

Also added a way to create a MeanPredictor based on an existing Predictor.

Issue #, if available:

Description of changes:

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[szha]

Job PR-240/1 is complete.
Docs are uploaded to http://gluon-ts-staging.s3-accelerate.dualstack.amazonaws.com/PR-240/1/index.html

[szha]

Job PR-240/2 is complete.
Docs are uploaded to http://gluon-ts-staging.s3-accelerate.dualstack.amazonaws.com/PR-240/2/index.html

[codecov-io]

Codecov Report

Merging #240 into master will decrease coverage by <.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
- Coverage   81.24%   81.24%   -0.01%     
==========================================
  Files         133      133              
  Lines        7429     7432       +3     
==========================================
+ Hits         6036     6038       +2     
- Misses       1393     1394       +1

Impacted Files	Coverage Δ
src/gluonts/model/trivial/identity.py	100% <100%> (ø)	⬆️
src/gluonts/model/predictor.py	82.35% <100%> (+0.18%)	⬆️
src/gluonts/model/trivial/constant.py	100% <100%> (ø)	⬆️
src/gluonts/model/trivial/mean.py	94.87% <84.61%> (-2.43%)	⬇️