MAINT: simplify base Transformer, move Transformer's __init__ and fit to MethodTF

[Zeroto521]

FeatureUnion also need Transformer's inverse_transform, but other attrs and methods are extra.

[codecov]

Codecov Report

Merging #192 (c422577) into master (78781e6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
- Coverage   99.34%   99.34%   -0.01%     
==========================================
  Files          20       20              
  Lines         460      457       -3     
  Branches       49       49              
==========================================
- Hits          457      454       -3     
  Misses          1        1              
  Partials        2        2              

Impacted Files	Coverage Δ
dtoolkit/transformer/base.py	100.00% <100.00%> (ø)

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