Apply node wrongly requires method parameter

[jonathanjfshaw]

AFAICS from the docs this should work:

    - id: my_apply
      module: timeflux.nodes.apply
      class: ApplyMethod
      params:
        func: abs

but it fails with Node 'my_apply': __init__() missing 1 required positional argument: 'method', because the signature is

    def __init__(self, method, apply_mode='universal',
                 axis=0, closed='right', func=None, **kwargs):

We should make method optional and raise an exception unless at least one of method and func are provided.