Retrieve all compiled variables in an SDD manager

[guyvdbroeck]

PySDD has a nice API: a, b, c, d = sdd.vars for retrieving all the compiled variables in the SDD manager. We currently have to build those variable circuits one by one. We should add this API.

[khosravipasha]

@guyvdbroeck I think we have this now. I guess can close this, unless something else was meant here

for example this from the readme.md

sun, rain, rainbow, cloud, snow, belgium, los_angeles = pos_literals(Sdd, manager, 7)