- these classes now encapsulate a std::shared_ptr<{const, }double>, so that
  they do not perform memory management, and several {Const,}Vector instances
  can transparently share the same underlying data

- make converting constructor from ConstVector to Vector explicit, since that
  entails memory allocation (but the reverse conversion is almost costless, so
  keep it implicit); do the same for GeneralMatrix/ConstGeneralMatrix,
  TwoDMatrix/ConstTwoDMatrix

- remove the constructors that were extracting a row/column from a matrix, and
  replace them by getRow() and getCol() methods on {Const,}GeneralMatrix

- rename and change the API of the complex version Vector::add(), so that it is
  explicit that it deals with complex numbers

- add constructors that take a MATLAB mxArray