Convert a conflict-free matrix to a tree object.
This function converts a conflict-free matrix to a tree object in which nodes are labled with cells and edges are lables with mutations. The root is labled by ‘root’. Mutations are seperated by
.graph['splitter_mut']and cells are seperated by
.graph['splitter_cell']. Those mutations that are not present in any cell are stored in
.graph['become_germline']. Mutations happed once during the evolution so there is no repetitive mutation.