[dartpy] Add optimizer APIs with GradientDescent and nlopt solvers

[jslee02]

Example code:

import math
import dartpy as dart


# Problem reference: http://ab-initio.mit.edu/wiki/index.php/NLopt_Tutorial
class SampleObjFunc(dart.optimizer.Function):
    def eval(self, x):
        return math.sqrt(x[1])

    def evalGradient(self, x, grad):
        grad[0] = 0
        grad[1] = 0.5 / (math.sqrt(x[1]) + 0.000001)


class SampleConstFunc(dart.optimizer.Function):
    def __init__(self, a, b):
        super(SampleConstFunc, self).__init__()
        self.a = a
        self.b = b

    def eval(self, x):
        a = self.a
        b = self.b
        return (a*x[0] + b) * (a*x[0] + b) * (a*x[0] + b) - x[1];

    def evalGradient(self, x, grad):
        a = self.a
        b = self.b
        grad[0] = 3 * a * (a*x[0] + b) * (a*x[0] + b);
        grad[1] = -1.0;


# Problem settings
prob = dart.optimizer.Problem(2)
prob.setLowerBounds([-1e100, 0])
prob.setInitialGuess([1.234, 5.678])
prob.setObjective(SampleObjFunc())
prob.addIneqConstraint(SampleConstFunc(2, 0))
prob.addIneqConstraint(SampleConstFunc(-1, 1))

# Solve
solver = dart.optimizer.NloptSolver(prob)
solver.setAlgorithm(dart.optimizer.NloptSolver.Algorithm.LD_MMA)
solver.solve()

# Solutions
min_f = prob.getOptimumValue()
opt_x = prob.getOptimalSolution()

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Before creating a pull request

• Document new methods and classes
• Format new code files using clang-format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[codecov]

Codecov Report

Merging #1325 into master will decrease coverage by 0.17%.
The diff coverage is 17.64%.

@@            Coverage Diff             @@
##           master    #1325      +/-   ##
==========================================
- Coverage    57.1%   56.93%   -0.18%     
==========================================
  Files         366      366              
  Lines       27050    27156     +106     
==========================================
+ Hits        15447    15460      +13     
- Misses      11603    11696      +93

Impacted Files	Coverage Δ
dart/optimizer/nlopt/NloptSolver.cpp	22.5% <17.64%> (-20.06%)	⬇️
dart/dynamics/Skeleton.cpp	66.12% <0%> (+0.16%)	⬆️
dart/dynamics/EulerJoint.cpp	70.73% <0%> (+3.04%)	⬆️