Add target names to Dataset

[relf]

This PR supersedes the PR #256 and complete the implementation. See also #70.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 19 lines in your changes missing coverage. Please review.

Project coverage is 35.37%. Comparing base (a30e5f1) to head (c67f244).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/dataset/impl_dataset.rs	57.57%	14 Missing ⚠️
...rithms/linfa-trees/src/decision_trees/algorithm.rs	50.00%	2 Missing ⚠️
src/dataset/iter.rs	60.00%	2 Missing ⚠️
src/correlation.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #373      +/-   ##
==========================================
+ Coverage   34.93%   35.37%   +0.44%     
==========================================
  Files          96       96              
  Lines        6334     6371      +37     
==========================================
+ Hits         2213     2254      +41     
+ Misses       4121     4117       -4     

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