Operations on geometries

We can categorise operations on geometries in terms of what they take as input and what they return as output: - predicates: a logical for a property TRUE
- measures
- transformations: newly generated geometries

In terms of what they operate on:
- unary: work on a single geometry
- is_simple()
- is_valid()
- is_empty()
- is_longlat()
- is(geometry, class)
- binary: work on pairs of geometries
- n-ary: work on sets of geometries