`label_dict` does not behave like a `dict`

[Helveg]

• Misses parts of the dict interface:
  • fromkeys
  • items
  • keys
  • values
  • pop
  • popitem
  • get
  • setdefault
  • update
  • clear
  • __reversed__
  • __contains__
• The arbor.label_dict function does not accept the same arguments as the dict constructor (e.g. doesn't know what to do with a zip object, it does not even accept another label_dict as arg)
• Converting a label_dict to a dict yields an empty dictionary, or an error:

  >>> labels = arbor.label_dict({'midpoint': '(location 0 0.5)'})
  >>> print(labels, dict(labels))
  Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  ValueError: dictionary update sequence element #0 has length 8; 2 is required

  (label_dict (region  "apic" (tag 4)) (region  "dend" (tag 3)) (region  "axon" (tag 2)) (region  "soma" (tag 1))) {}

[thorstenhater]

Implemented in #1711

• items, keys, values
• contains
• update
• conversion from iterator of kv pairs
• copy from other label_dict

As label_dict is essentially frozen, we cannot implement

• pop, popitem
• clear

These remain open/untested, but I am less sure we need all of them

• get: covered by []?
• setdefault: Is this required/needed?
• reversed: As this is essentially an unordered container, has this value?
• conversion to dict: Should be ok by converting to iterator

@Helveg does this fulfill your needs?

[Helveg]

• get accepts a default argument that simplifies control flow:

val = labels.get("key", "(nil)")
try:
  val = labels["key"]
except KeyError:
  val = "(nil)"

• Same for setdefault
• I don't see any need for reversed
• Would dict(label_dict) work? With an incomplete dict API it can't be duck typed everywhere so the conversion might occur.

Why not implement __delitem__? Imagine wanting to strip a certain set of keys from a factory label dict. I don't see a specific use case for pop(item) since it is unordered, but del certainly.