ENH: additional polynomial constructors, improved I/O

- support construct from initializer_list, which can help simplify
  code with constant coefficients.

- add default constructor for polynomialFunction and Istream reading
  to support resizable lists of polynomialFunction.

  A default constructed polynomialFunction is simply equivalent to
  a constant zero.

- no special IO handling for Polynomial required,
  it is the same as VectorSpace anyhow.