[Bug] `id` should be compulsory in `Instance`

[sanjayankur31]

id in Instance should be required, but is currently not because we can either provide an id or i, j, k locations. This condition is not currently included in the schema, so if someone misses the id, but does not give i, j, k values, the model is still valid---but will throw cryptic errors when LEMS tries to parse/simulate it.

We intend to drop regions etc. soon, and that'll also allow us to drop the i, j, k attributes here and make id a required attribute.