Rework `AND` and `OR` dummy structure

[IbrahimElk]

When using model counter to compile a sd-DNNF circuit using klay,
the resulting cirucit contains a lot of dummy nodes.

(gdb) pcirc circuit
A3/0
├── O2/2
│   └── A1/0
│       └── L0/2  (x1)
└── O2/1
    ├── A1/2
    │   └── L0/4  (x2)
    └── A1/3
        └── L0/5  (¬x2)

These can be pruned to be like the following:

(gdb) pcirc circuit
A3/0
├── L0/2       (x1) 
│ 
└── O2/1
    ├──  L0/4  (x2)
    │
    └──  L0/5  (¬x2)

As far as i'm told, @rmanhaeve and @jjcmoon are working on this in the rework branch ?

[rmanhaeve]

This seems to be a consequence of forcing the first layer to be an AND. This is indeed something I was considering changing in stratification-rework, but haven't gotten further into that yet as it's quite a substantial assumption in the code.