mimic std::optional for flagged Waypoint fields. (#975)

mimic std::optional for flagged Waypoint fields. (#975)

* mimic std::optional for flagged Waypoint fields.

* manual adjustments to waypt access usage.

* split optional Waypoint flags, tune packing.

* bring op_flags into Waypoint.

* add comments about future Waypoint optional fields.

* sort Waypoint class almost normally.

private members refered to with decltype need to be defined
before they are referred to.

* remove extra parens around *has_value().

* remove extra parens around *value_or().

* remove more extra parens.