Add `feature_labels` to `BaseDescriptor`

[TsumiNa]

Close #211

Because BaseDescriptor can accept multiple inputs based on how the featurizers are organized, property feature_labels also has two behaves.

When only one group of featurizers in BaseDescriptor, feature_labels returns a list exactly like the BaseFeaturizer do. When more than one groups, feature_labels returns a tuple as:

(('<group1_name>', [<concatenated_labels>]), ('<group2_name>', [<concatenated_labels>]), ...)

[codecov[bot]]

Codecov Report

Merging #212 (cf4ab7d) into master (767d015) will decrease coverage by 48.17%. The diff coverage is 3.56%.

@@             Coverage Diff             @@
##           master     #212       +/-   ##
===========================================
- Coverage   82.40%   34.22%   -48.18%     
===========================================
  Files          93       95        +2     
  Lines        6480     6915      +435     
===========================================
- Hits         5340     2367     -2973     
- Misses       1140     4548     +3408     

Impacted Files	Coverage Δ
xenonpy/__init__.py	0.00% <0.00%> (-73.53%)	:arrow_down:
xenonpy/_conf.py	0.00% <0.00%> (-93.11%)	:arrow_down:
xenonpy/contrib/extend_NGram/__init__.py	0.00% <0.00%> (ø)
xenonpy/contrib/extend_NGram/new_ngram.py	0.00% <0.00%> (ø)
...riptors/descriptor/frozen_featurizer_descriptor.py	0.00% <0.00%> (-43.75%)	:arrow_down:
xenonpy/descriptor/base.py	0.00% <0.00%> (-91.41%)	:arrow_down:
xenonpy/model/training/checker.py	0.00% <0.00%> (-84.28%)	:arrow_down:
xenonpy/model/training/extension/persist.py	0.00% <ø> (-75.95%)	:arrow_down:
tests/datatools/test_dataset.py	99.10% <100.00%> (ø)
tests/descriptor/test_base_desc.py	98.64% <100.00%> (+0.05%)	:arrow_up:
... and 65 more

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[TsumiNa]

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