Eigen 3.4 adds std::initializer_list constructors

[RMeli]

Eigen 3.4 adds std::initializer_list constructors and this does not play nicely with the custom constructor defined in libirc/plugins/eigen/.

Trying to compile with Eigen 3.4 results in the following error, given that there are now two constructors with the same signature:

/Users/rmeli/Documents/git/projects/irc/src/test/wilson_test.cpp:92:9: error: call to constructor of 'const Eigen::Matrix<double, -1, -1, 0>' is ambiguous
        {// Rotation for H1
        ^~~~~~~~~~~~~~~~~~~
/usr/local/include/eigen3/Eigen/src/Core/Matrix.h:321:34: note: candidate constructor
    explicit EIGEN_STRONG_INLINE Matrix(const std::initializer_list<std::initializer_list<Scalar>>& list) : Base(list) {}
                                 ^
/Users/rmeli/Documents/git/projects/irc/include/libirc/plugins/eigen/Matrix_initializer_list.h:60:21: note: candidate constructor
EIGEN_STRONG_INLINE Matrix(std::initializer_list<std::initializer_list<_Scalar>> initlist) : Base()
                    ^
/Users/rmeli/Documents/git/projects/irc/src/test/wilson_test.cpp:96:9: error: call to constructor of 'const Eigen::Matrix<double, -1, -1, 0>' is ambiguous
        {// Rotation for O
        ^~~~~~~~~~~~~~~~~~
/usr/local/include/eigen3/Eigen/src/Core/Matrix.h:321:34: note: candidate constructor
    explicit EIGEN_STRONG_INLINE Matrix(const std::initializer_list<std::initializer_list<Scalar>>& list) : Base(list) {}
                                 ^
/Users/rmeli/Documents/git/projects/irc/include/libirc/plugins/eigen/Matrix_initializer_list.h:60:21: note: candidate constructor
EIGEN_STRONG_INLINE Matrix(std::initializer_list<std::initializer_list<_Scalar>> initlist) : Base()
                    ^
/Users/rmeli/Documents/git/projects/irc/src/test/wilson_test.cpp:100:9: error: call to constructor of 'const Eigen::Matrix<double, -1, -1, 0>' is ambiguous
        {// Rotation for H2
        ^~~~~~~~~~~~~~~~~~~
/usr/local/include/eigen3/Eigen/src/Core/Matrix.h:321:34: note: candidate constructor
    explicit EIGEN_STRONG_INLINE Matrix(const std::initializer_list<std::initializer_list<Scalar>>& list) : Base(list) {}
                                 ^
/Users/rmeli/Documents/git/projects/irc/include/libirc/plugins/eigen/Matrix_initializer_list.h:60:21: note: candidate constructor
EIGEN_STRONG_INLINE Matrix(std::initializer_list<std::initializer_list<_Scalar>> initlist) : Base()
                    ^
/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/vector:566:41: note: passing argument to parameter '__il' here
    vector(initializer_list<value_type> __il);
                                        ^
/Users/rmeli/Documents/git/projects/irc/src/test/wilson_test.cpp:170:15: error: call to constructor of 'const mat' (aka 'const Matrix<double, Dynamic, Dynamic>') is ambiguous
    const mat R{{cos(angle_rad), -sin(angle_rad), 0},
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/eigen3/Eigen/src/Core/Matrix.h:321:34: note: candidate constructor
    explicit EIGEN_STRONG_INLINE Matrix(const std::initializer_list<std::initializer_list<Scalar>>& list) : Base(list) {}
                                 ^
/Users/rmeli/Documents/git/projects/irc/include/libirc/plugins/eigen/Matrix_initializer_list.h:60:21: note: candidate constructor
EIGEN_STRONG_INLINE Matrix(std::initializer_list<std::initializer_list<_Scalar>> initlist) : Base()
                    ^
4 errors generated.