Define an standard syntax for Backwards Vertical Fusion

[morousg]

We demonstrated the possibility of performing Backwards Vertical Fusion, but it's implementation it's not defined in an estandard way.

We want to add a ReadBackwardsType, that has the same parameters as ReadType, plus an extra one that will be a ReadDeviceFunction or a ReadBackwardsDeviceFunction, that the ReadBackwardsDeviceFunction will use to read each of the elements it works with.