Fix nds

[juliambr]

• Fixed a bug: If reference point is given, it has also be transformed into a minimization problem
• Replaced computation of hypervolume contribution by the implementation in emoa
• Fixed a bug in the the sampling in case two or more points have the same hypervolume contribution.
• Re-write of tests, because they have been a bit lengthy

[codecov-commenter]

Codecov Report

Merging #67 into master will decrease coverage by 1.11%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
- Coverage   91.86%   90.75%   -1.12%     
==========================================
  Files           3        3              
  Lines         123      119       -4     
==========================================
- Hits          113      108       -5     
- Misses         10       11       +1     

Impacted Files	Coverage Δ
R/nds_selection.R	96.00% <85.71%> (-4.00%)	:arrow_down:

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[jakob-r]

Looks good!