Fix data flow during `fit_transform`

[alex-hse-repository]

🚀 Feature Request

The current version of data flow during fit_transform is not optimal, it might be improved.

Proposal

1. In to_pandas method:

• Add flag copy:bool=True, implement the corresponding logic
• Remove segments from slices

2. In BaseTransform class:

• Method fit should request the reference to the full df
• Methods transform/inverse_transform should request the copy of the df with required features

Test cases

1. Test that flag copy works, get the dataframe, change it and compare with dataframe in dataset
2. Fix tests for BaseTransform if nessesary

Additional context

No response