0.6.2

New Features

• Add C++ ExpNode and Python Exp symbol. Exp propagates the
  base-e exponential values of its predecessor element-wise. See
  #282.
• Add Input symbol and corresponding InputNode node in the C++.
  This symbol is meant to function as a "placeholder" in a model.
• Improve the error messages returned by ListVariable.set_state()
  and SetVariable.set_state().
• Add C++ CollectionNode::assign() method.
• Add absolute function as an alias for the built-in abs.
• Add Python safe_divide() function and SafeDivide symbol. Also
  add C++ SafeDivideNode. See
  #290.
• Add overloads to C++ methods Graph::commit(),
  Graph::propagate(), and Graph::revert() that commit, propagate,
  or revert respectively all nodes in the graph.
• Add a default constructor for C++ BinaryNode.

Upgrade Notes

• Rename the first argument ListVariable.set_state() and
  SetVariable.set_state() from state to values.

Bug Fixes

• Make C++ dwave::optimization::functional objects more consistent
  with the standard library by adding overloads for other numeric
  types and marking most operations as constexpr.
• Fix iteration over ArraySymbols. Now iteration works correctly for
  ArraySymbols with more than one dimension and a fixed size. Other
  ArraySymbols now raise a TypeError. See
  #287.