validate definition JSON

We should validate the definition JSON and it's parts (`datasets`, `series`, `type`, etc) etc. For example:
- each dataset needs a unique `name` property, and _either_ a `url` or `data` property
- each `series` needs a `source` property that refers to an existing dataset
- `type` should be one of the supported types or 'custom'

When do we run this validation? In the setters? That might mean there is an implicit sequence in which you have to set things (i.e. you can't set `series` before datasets). Or validate lazily (i.e. first thing when calling `.show()` or it's sub methods)?