Initialize Compare with (a list of) features

[jpweytjens]

A small fix for #117. A Compare object set self.features to an empty list after initialization. This is removed so self.features is correctly initialized.

The documentation mentions that a both individual features or a list of features can be added. The add method always appends, leading to nested lists. This PR extends self.features if a list is passed, and appends otherwise.

[J535D165]

Thanks Johannes! I corrected a breaking bug and added a test (that revealed a new bug). I think this one is ready for merging.

Closes #117

[codecov]

Codecov Report

Merging #124 into master will increase coverage by 1.36%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage    77.7%   79.07%   +1.36%     
==========================================
  Files          34       34              
  Lines        2476     2676     +200     
  Branches      392      442      +50     
==========================================
+ Hits         1924     2116     +192     
- Misses        419      421       +2     
- Partials      133      139       +6

Impacted Files	Coverage Δ
recordlinkage/base.py	80.77% <100%> (+0.15%)	⬆️
recordlinkage/classifiers.py	85.54% <0%> (+1.61%)	⬆️
recordlinkage/algorithms/nb_sklearn.py	78.74% <0%> (+11.77%)	⬆️

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[jpweytjens]

Thanks for spotting the wrong order of arguments.