Determine if a buffer is not needed for decompression of acyclic coloring

[amontoison]

With #180, we can now use the vector of booleans is_star to determine if all trees are stars or not.
If it is the case, we don't need to allocate a buffer for the decompression.

[amontoison]

Not relevant in practice.
We don't need a buffer only if the acyclic coloring computed is a star coloring (very rare cases).