Angles have their own system that define both radians and degrees.  I would like to create the necessary type for minutes (this is 1/60 of a degree).  My first thought was to create a scale unit with degrees as the base type.  I couldn't determine how to enter the scale of 1/60.  The other option is to define a new base unit.  I would think using the degrees base unit would be a good reference, but it uses an "INTERNAL ONLY" macro call "BOOST_UNITS_DEFINE_BASE_UNIT_WITH_CONVERSIONS".  What should I use to create a unit type that will convert from degree minutes to just degrees?