Add class method to add disambiguation

[librasteve]

This gives type => Length, Reach

Custom Measures
To make a custom Measure, you can use this incantation:

GetMeaUnit('nmile').NewType('Reach');

class Reach is Measure {
    has $.units where *.name eq <nm nmile nmiles>.any;

    #| override .in to perform identity 1' (Latitude) == 1 nmile
    method in( Str $s where * eq <Latitude> ) { 
        my $nv = $.value / 60; 
        Latitude.new( value => $nv, compass => <N> )
    }   
}