fix inverse_transform in encoders

[solegalli]

In addition:

• add functionality to encode unseen in ordinal encoder
• change logic of encoding unseen categories
• increase code coverage for encoders
• remove CategoricalMixinExpanded
• remove dictionary check in fit
• add deprecation warning in PRatioEncoder

closes #428
closes #538

[solegalli]

FYI @glevv

[codecov]

Codecov Report

Merging #539 (ed35133) into main (93cb37b) will increase coverage by 0.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #539      +/-   ##
==========================================
+ Coverage   96.84%   97.39%   +0.54%     
==========================================
  Files          88       88              
  Lines        3331     3336       +5     
  Branches      671      655      -16     
==========================================
+ Hits         3226     3249      +23     
+ Misses         45       32      -13     
+ Partials       60       55       -5     

Impacted Files	Coverage Δ
feature_engine/encoding/base_encoder.py	98.59% <100.00%> (+11.57%)	⬆️
feature_engine/encoding/count_frequency.py	96.87% <100.00%> (-0.10%)	⬇️
feature_engine/encoding/decision_tree.py	96.07% <100.00%> (+1.96%)	⬆️
feature_engine/encoding/mean_encoding.py	100.00% <100.00%> (+4.76%)	⬆️
feature_engine/encoding/one_hot.py	100.00% <100.00%> (+4.76%)	⬆️
feature_engine/encoding/ordinal.py	97.22% <100.00%> (+0.44%)	⬆️
feature_engine/encoding/probability_ratio.py	90.38% <100.00%> (+0.80%)	⬆️
feature_engine/encoding/rare_label.py	100.00% <100.00%> (+6.12%)	⬆️
feature_engine/encoding/woe.py	100.00% <100.00%> (ø)
... and 3 more

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