Add transpose operator

[pditommaso]

The transpose operator implements the opposite function of groupTuple. For example given a channel emitting the following tuples:

[ key1, [value1, value2] ] 
[ key2, [value3, value4] ] 

apply transpose it will return the following:

[ key1, value1 ]
[ key1, value2 ] 
[ key2, value3 ] 
[ key2, value4 ]

[pditommaso]

More complex examples:

[ a, [p,q], [x,y] ]
[ b, [s,t], [x,y] ] 

the transpose returns:

[ a, p, x ] 
[ a, q, y ] 
[ b, s, x ] 
[ b, t, y ] 

given:

[ a, [p,q], [x,y,w,z] ]
[ b, [s,t], [x,y,w,z] ] 

It returns:

[ a, p, x ]
[ a, q, y ]
[ b, s, x ]
[ b, t, x ]

For the same input, specifying remainder: true it returns:

[ a, p, x ]
[ a, q, y ]
[ a, null, w ]
[ a, null, z ]
[ b, s, x ]
[ b, t, x ]
[ b, null, w ]
[ b, null, z ]

[pditommaso]

Included in version 0.26.0-beta3