Filtering with a Migration

@KevinDCarlson I was reading the tests and I don't see why this migration filters things. Shouldn't I have to assert that d == l somewhere?

```julia
#Filter impossible edges out of a mechanical linkage
M = @migration SchMechLink SchMechLink begin
V => V
E => @join begin
e :: E
L :: Len
(l:e→L) :: (x->len(x)^2)
(d:e→L) :: (x->sum((pos(src(x))-pos(tgt(x))).^2))
end
Pos => Pos
Len => Len
src => src(e)
tgt => tgt(e)
pos => pos
len => len(e)
end
```