Clarify Unit API power units and add derived units

[hongted]

Currently the OpenSTA API supports querying a certain set of units including time, cap, voltage, power, etc...

OpenSTA/include/sta/Units.hh

Lines 68 to 102 in 9c9b565

	// User interface units.
	// Sta internal units are always seconds, farads, volts, amps.
	class Units
	{
	public:
	Units();
	Unit *find(const char *unit_name);
	void operator=(const Units &units);
	Unit *timeUnit() { return &time_unit_; }
	const Unit *timeUnit() const { return &time_unit_; }
	Unit *capacitanceUnit() { return &capacitance_unit_; }
	const Unit *capacitanceUnit() const { return &capacitance_unit_; }
	Unit *voltageUnit() { return &voltage_unit_; }
	const Unit *voltageUnit() const { return &voltage_unit_; }
	Unit *resistanceUnit() { return &resistance_unit_; }
	const Unit *resistanceUnit() const { return &resistance_unit_; }
	Unit *currentUnit() { return &current_unit_; }
	const Unit *currentUnit() const { return &current_unit_; }
	Unit *powerUnit() { return &power_unit_; }
	const Unit *powerUnit() const { return &power_unit_; }
	Unit *distanceUnit() { return &distance_unit_; }
	const Unit *distanceUnit() const { return &distance_unit_; }
	Unit *scalarUnit() { return &scalar_unit_; }
	const Unit *scalarUnit() const { return &scalar_unit_; }
	private:
	Unit time_unit_;
	Unit resistance_unit_;
	Unit capacitance_unit_;
	Unit voltage_unit_;
	Unit current_unit_;
	Unit power_unit_;
	Unit distance_unit_;
	Unit scalar_unit_;
	};

This issue is a API enhancement request to

1. Rename the existing powerUnit() to leakagePowerUnit() for clarity
2. Add additional Units for derived values such as dynamicPowerUnit() and energyUnit().