Add option to collapse low support branches

[peterk87]

With ete3, it is easy to delete/collapse internal branches based on support value:

from ete3 import Tree, TreeNode

tree = Tree()
A = tree.add_child(name='A', support=100)
A1 = A.add_child(name='A1')
A2 = A.add_child(name='A2')
B = tree.add_child(name='B', support=50)
C = B.add_child(name='C')
D = B.add_child(name='D')
print(tree.get_ascii(show_internal=True))

#       /-A1
#    /A|
#   |   \-A2
# --|
#   |   /-C
#    \B|
#       \-D

# traversing nodes to remove internal nodes with support less than 95
for node in tree.traverse():
    if not node.is_leaf() and not node.is_root():
        if node.support < 95:
            node.delete()
print(tree.get_ascii(show_internal=True))

#       /-A1
#    /A|
#   |   \-A2
# --|
#   |--C
#   |
#    \-D

[peterk87]

Fixed in #5