The elevation query API is no longer tied to Maps. It is abstracted in
the `ElevationProvider` interface. This makes it possible to implement
elevation query APIs with other entities for example.

Additionally, the `aggregateElevationProviders()` function is introduced
and allows merging several providers into one, so that we can use a
single entry point to query multiple maps at the same time for example.

This could be useful in scenes that contain overlapping terrains.

BREAKING CHANGE: The `GetElevationOptions`, `GetElevationResult` and
`ElevationSample` types are moved from `entities` to `core` namespace.
`ElevationSample.map` is renamed `ElevationSample.source` and its type
is now `ElevationProvider`:

```ts
// Before
import '@giro3d/giro3d/entities/GetElevationResult';

// Now
import '@giro3d/giro3d/core/GetElevationResult';
````

fix #519