Refactor serialization

[atmoos]

Serialization is broken for inverted quantities. As an elegant solution, we should change the format of non-compound quantities to something resembling:

{
  "length": {
    "value": 4.2,
    "exponent": 1,
    "si": {
      "unit": "m"
    }
  }
}

The key being the added exponent, which simplifies both power measures (m³, etc.) but also inverse measures.

[atmoos]

Consider declaring the exponent optional. Meaning that an absent value means exponent: 1.