We now use a initializer list constructor for creating symmetries of the form
$y^n$, $y^n u^m$, $y^nu^m\sigma^k$.

The constructor taking a single integer is used to initialize a symmetry of a
given length.

Similar changes are made to IntSequence.

This behavior is similar to std::vector.

- 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

Performed using modernize-use-nullptr from clang-tidy.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

Performed using modernize-use-auto from clang-tidy.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-auto.html

Performed using modernize-loop-convert from clang-tidy.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-loop-convert.html