Unit convert

Convert a value between units of length, mass, temperature, volume, area, speed, time, data, pressure, energy, power, angle, frequency - every unit the retired convert-* endpoints handled (e.g. miles, kilometers, kilograms, pounds, stones, us-gallons, liters, fahrenheit, psi, bar, kilopascals, kilowatt-hours, joules, btu, horsepower, and imperial Russian versts, sazhens, arshins, poods and funts).