Duplicate DistanceMatrix code in eigen_helpers.hpp and arc_helpers.hpp

[dmcconachie]

eigen_helpers.hpp

template<typename ScalarType, int VectorLength, int NumVectors>              
inline Eigen::Matrix<double, NumVectors, NumVectors> CalculateSquaredDistanceMatrix(const Eigen::Matrix<ScalarType, VectorLength, NumVectors>& set)

arc_helpers.hpp:

template<typename Datatype, typename Allocator = std::allocator<Datatype>>   
inline Eigen::MatrixXd BuildDistanceMatrix(const std::vector<Datatype, Allocator>& data, const std::function<double(const Datatype&, const Datatype&)>& distance_fn)

template<typename FirstDatatype, typename SecondDatatype, typename FirstAllocator = std::allocator<FirstDatatype>, typename SecondAllocator = std::allocator<SecondDatatype>>
inline Eigen::MatrixXd BuildDistanceMatrix(const std::vector<FirstDatatype, FirstAllocator>& data1, const std::vector<SecondDatatype, SecondAllocator>& data2, const std::function<double(const                 FirstDatatype&, const SecondDatatype&)>& distance_fn)

We might want to consider cleaning this up at some point.