JSON: Selectively suppress unknown key warnings

[franzpoeschel]

Example use case: This came up when in the last release we added the key adios2.engine.attribute_writing_ranks. When adding this key in downstream codes (came up in PIConGPU), then that code might either be compiled against the recent version which knows that key or against the older version which knows it not.
In the latter case, a runtime warning about an unknown key will be printed. To avoid that issue, this PR adds a flag to opt-out keys from warnings:

{
  "dont_warn_unused_keys": [
    "key1",
    "key3"
  ],
  "key1": 5,
  "key2": {
    "dont_warn_unused_keys": [
      "key1"
    ],
    "key1": 5,
    "key2": 6
  },
  "key3": {
    "dont_warn_unused_keys": [
      "key1"
    ],
    "key1": 5,
    "key2": 6
  }
}

Will yield the warning:

[Series] The following parts of the global JSON config remains unused:

{
  "key2": {
    "dont_warn_unused_keys": [
      "key1"
    ],
    "key1": 5,
    "key2": 6
  },
  "key3": {
    "key2": 6
  }
}

TODO:

• better errors upon parsing wrong configurations
• documentation
• testing (can we check if a test writes to stderr?)