add helper fn to create process chains

[metzm]

Maybe this new helper function is of wider interest, e.g. for the actinia-python-client.

How to use:

1. initialize a process chain

   process_chain = {"version": 1, "list": []}

   2. create a new item for the list

      pc_item = create_actinia_pc_item(...)

2. add the new item to the list

   process_chain["list"].append(pc_item)

3. Repeat 2. and 3. until the list is complete, then send the process chain to actinia

   request = requests.post(url=request_url, auth=actinia_auth, json=process_chain)