Potential DAG (regression?) "bug": name unicity not enforced!

[thorwhalen]

Try playing with this code:

from meshed import DAG, FuncNode

def foo(x):
    return x + 1

t = foo

def foo(y):
    return y * 2

tt = foo

d = DAG([t, tt])
d.func_nodes
f, ff = d.func_nodes
f.func_label, ff.func_label

[thorwhalen]

I think I closed this by mistake trying to close #45.

Reopening, but @valentin-feron and @sylvainbonnot see if you agree that it's still an issue.