Add tensor overloaded operators for component-wise operations

[fredrikbk]

Functions:

/// Negate tensor
template <typename C>
Expr operator-(const Tensor<C>&) {
  taco_not_supported_yet;
  return Expr();
}

/// Scale tensor
template <typename C>
Expr operator*(const Tensor<C>&, C) {
  taco_not_supported_yet;
  return Expr();
}

/// Scale tensor
template <typename C>
Expr operator*(C, const Tensor<C>&) {
  taco_not_supported_yet;
  return Expr();
}

/// Add tensors
template <typename T>
Expr operator+(const Tensor<T>&, const Tensor<T>&) {
  taco_not_supported_yet;
  return Expr();
}

/// Subtract tensors
template <typename T>
Expr operator-(const Tensor<T>&, const Tensor<T>&) {
  taco_not_supported_yet;
  return Expr();
}

Methods:

  Tensor<C>& operator*=(C) {
    taco_not_supported_yet;
  }

  Tensor<C>& operator+=(const Tensor<C>&) {
    taco_not_supported_yet;
  }

  Tensor<C>& operator-=(const Tensor<C>&) {
    taco_not_supported_yet;
  }