* increase level of leveled node by level, default `level` equals `1`
* if `totallevel > maxlevel`, returns rest (`total-max`)
* can be negative for decreasing
-* `nodeupdate_single(pos)`
- * causes an unsupported `group:falling_node` node to fall and causes an
- unattached `group:attached_node` node to fall.
- * does not spread these updates to neighbours
- * a helper function, not officially part of the API, but useful
### Inventory
`minetest.get_inventory(location)`: returns an `InvRef`