sequence function: distance

    description:
        distance(ket, ket)
        the Euclidean distance function
        
    the algorithm:
        r = sqrt((x1 - y1)^2 + (x2 - y2)^2 + ... + (xn - yn)^2 )
        
    examples:
        -- without categories:
        distance(|0: 0>, |0: 7>)
            |number: 7>
            
        -- using categories, and a Pythagorean triple:
        distance(|grid: 0: 0>, |grid: 3: 4>)
            |number: 5>
        
        -- if categories are different, return don't know ket:
        distance(|grid: 0: 0>, |position: 3: 4>)
            |>

        -- another Pythagorean triple:
        distance(|grid: 0: 0>, |grid: 5: 12>)
            |number: 13>
        
        -- sqrt(2):
        distance(|0: 0>, |1: 1>)
            |number: 1.414>
        
        distance(|0: 0>, |1: 2>)
            |number: 2.236>        
           
    see also:
        simm 

    future:
        maybe change the number of decimal points in the result, currently set to 3
        maybe remove the number prefix too?