Add method to transformer

[BalzaniEdoardo]

Make sure that TransformerBasis has a to_transformer method. it may happen that, while scripting, one forgot if a basis is already a transformer and call to_transformer multiple times. This prevents exception to be raised and de-facto, does a deep copy of the transformer.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.92%. Comparing base (dc72fcd) to head (ee23844).
⚠️ Report is 114 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #427      +/-   ##
===============================================
- Coverage        72.47%   69.92%   -2.55%     
===============================================
  Files               97      104       +7     
  Lines             9413     9826     +413     
===============================================
+ Hits              6822     6871      +49     
- Misses            2591     2955     +364     

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