Add a way to deal with negative branch lengths affecting plotting

[oschwery]

• negative BLs mess up fastAnc (does sqrt(BL), which leads to NAs)
• NAs then mess the plot up downstream
• either use something BL independent instead of fastAnc
• or add something like tree$edge.length <- tree$edge.length + abs(min(tree$edge.length))
• or throw an error and let user deal with it based on own needs