Implement new mixture for single cell likelihood

[junseonghwan]

The independence assumption across sites cannot capture branching vs linear structure.

When assigning cell to node v,

• Split the nodes into 2 sets:
  • U: ancestral nodes.
  • W: non-ancestral nodes.
• Build conditional distribution that conditions on M_W = max{ b_w : w \in W} as well as progression along the ancestral nodes u \in U.

[junseonghwan]

Simpler approach is to construct the likelihood as mixture of 3 distributions one for dropout, burst for variant allele, and bi-allelic expression.