Make it possible to override text (atom) node with an array or an object node

[dakusui]

Currently if a text node and an object (or an array) node are found at the same path during inheritance processing, an error will be reported.
This behavior is not sufficiently flexible if we want to define a following node in order to force users to define a certain node.

base.json

{
    "mandatoryNode": "eval:$(error Please define this node!)"
}

Because the following file results in an error.

test.json

{
    "$extends": [ "base.json" ],
    "mandatoryNode": {
      "attr": "value"
    }
}

We want the following feature.
In case type mismatch happens, to an object node, an entry "$type": "object".
If it is an atom node, instead, the node will be converted into the following one.

{
    "$type": "atom",
    "$atomValue": "original atom value comes here"
}

If it is an array,

{
    "$type": "array",
    "$arrayValue": ["original", "values", "come", "here"]
}

If it is an object,

{
     "$type": "object",
     "originalKey1": "originalValueForOriginalKey1StaysHere",
     "originalKey2": "originalValueForOriginalKey2StaysHere",
    "...": "..."
}

Then merging will be performed.
But, note that

• $atomValue will be removed before the "templating"(evaluation) stage is executed, unless the $type is atom.
• Similarly, $arrayValue will be removed, unless the $type is array.