Implement matrix construction from an initializer list of initializer lists, if possible

[demianmnave]

From CML1 created by demianmnave : demianmnave/CML1#4

Constructing a matrix using initializer_list syntax would be natural and pretty convenient, e.g.:

matrix33d M({ { 1., 2., 3. }, { 4., 5., 6. }, { 7., 8., 9. } });

or

matrixd M = { { 1., 2., 3. }, { 4., 5., 6. }, { 7., 8., 9. } };