preliminary UnionNode implementation

[Jasha10]

This PR implements support for unions of primitive types (i.e. unions of int, float, bool, str, bytes, and None).

A few notes:

• Union can be freely mixed with Optional and None. For example, all of the following are equivalent:
  • Optional[Union[int, str]]
  • Union[Optional[int], str]
  • Union[int, str, None]
  • Union[int, str, type(None)]
• Containers of unions (e.g. List[Union[int, str]], or Dict[int, Union[str, bytes]]) are supported.
• Unions of containers (e.g. Union[List[str], Dict[str, str], MyStructuredConfig]) are not yet supported. This will come in a followup PR.

Here is a demo of the API:

from omegaconf import OmegaConf, ValidationError
from dataclasses import dataclass
from typing import Union
from pytest import raises
@dataclass
class Foo:
    u: Union[int, str] = 123

cfg = OmegaConf.create(Foo)
assert cfg.u == 123
cfg.u = "abc"  # ok

with raises(ValidationError):
    cfg.u = b"binary"  # bytes not compatible with union

with raises(ValidationError):
    OmegaConf.create(Foo(1.2))  # float not compatible

Implementation Notes:

• UnionNode is a "pass-through" node. That means most validation logic is delegated to a child node.
• I've introduced a new method Container._get_child which parallels Container._get_node. The difference between these methods is that _get_child automatically "passes over" Unions. Similarly, I've introduced Node._get_parent_container(), which is similar to Node._get_parent() except that it passes over union nodes in the parent heirarchy.

from dataclasses import dataclass
from typing import Dict, Union
from omegaconf import MISSING, OmegaConf, StringNode, UnionNode

@dataclass
class Example:
    cfg: Dict[str, Union[str, int]] = MISSING

cfg = OmegaConf.create(Example({"foo": "bar"})).cfg

assert isinstance(cfg._content, dict)
assert isinstance(cfg._content["foo"], UnionNode)
assert isinstance(cfg._content["foo"]._content, StringNode)
assert isinstance(cfg._content["foo"]._content._val, str)
assert cfg._content["foo"]._content._val == "bar"

assert cfg._content["foo"]._metadata.ref_type == Union[int, str]
assert cfg._content["foo"]._content._metadata.ref_type == str

assert cfg._get_node("foo") is cfg._content["foo"]            # UnionNode
assert cfg._get_child("foo") is cfg._content["foo"]._content  # StringNode

TODO before merging this

• docs
• serialization test
• Double-check usage of _get_parent vs _get_parent_container throughout the codebase.
• Double-check usage of _get_node vs _get_child throughout the codebase.

[pixelb]

The Box abstraction is great, and make things very neat.
The implementation all looks good to me.

In general splitting out the union of containers support until later, is fine and good.

Pre-approving assuming the outstanding tasks are done.

Thanks!