Always use binary tree with a root

[Karel Břinda]

MashTrees may be unresolved and have no root, e.g., the test example:

(GCGS0003:0.00008,(GCGS0004:0.00008,GCGS0002:0.00017):0.00097,(GCGS0005:0.00004,GCGS0001:0.00004):0.00005);