add sensible defaults

[Plutoberth]

from google sheets.
example json:

{
    "costs": {
        "solar": {
            "capex": [
                {"start_year": 2020,
                 "end_year": 2050,
                 "interpo": {
                    "type": "constant",
                    "value": 2030
                 }}
            ]
        }
    }
}

[Plutoberth]

Done by @sheyeh :)