Add Optimizer and objective_function

[be-marc]

Closes #78 and #33

• Adds OptimizerGenSA, OptimizerGridSearch and OptimizerDesignPoints
• Adds objective_function to OptimInstance

[codecov-commenter]

Codecov Report

Merging #79 into master will increase coverage by 0.66%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   96.96%   97.63%   +0.66%     
==========================================
  Files          22       25       +3     
  Lines         363      422      +59     
==========================================
+ Hits          352      412      +60     
+ Misses         11       10       -1     

Impacted Files	Coverage Δ
R/OptimizerRandomSearch.R	100.00% <ø> (ø)
R/OptimInstance.R	97.77% <100.00%> (+0.55%)	:arrow_up:
R/OptimizerDesignPoints.R	100.00% <100.00%> (ø)
R/OptimizerGenSA.R	100.00% <100.00%> (ø)
R/OptimizerGridSearch.R	100.00% <100.00%> (ø)
src/is_dominated.c	100.00% <0.00%> (+3.57%)	:arrow_up:

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[be-marc]

Also, please add a test for OptimInstanceMultcrit$objective_function()

There is already one.