Add helper function operator.create_filter()

_From @fishroot on December 17, 2018 7:41_

e.g.
```
p1 = lambda seq: seq[0] > seq[1]
p2 = lambda seq: isinstance(seq[2], str)
op = operator.create_filter(p1, p2)
```

_Copied from original issue: frootlab/nemoa#573_