`<include>` should only be allowed in `<world>` or `<model>`

## Environment
* OS Version: all
* Source or binary build? all

## Description
* Expected behavior: Using the `` tag in elements other than `` and `` should emit errors and not try to insert the included file as a child of any parent element. The `` tag is defined in the `world` and `model` spec files.
* Actual behavior: No error is emitted when using `` in elements such as `` or ``.

## Steps to reproduce
1. Save the following to `include_test.sdf`:
```xml

test_model

```
1. Set `SDF_PATH` to the directory containing `test_model`
```
export SDF_PATH=/test/integration/model
```

1. Run `gz sdf -k include_test.sdf`. The output is `Valid.`
1. Run `gz sdf -p include_test.sdf`. The output is the following:
```xml

meshes/mesh.dae

my_submesh
true

0.10000000000000001 0.20000000000000001 0.29999999999999999






meshes/mesh.dae

another_submesh
false

1.2 2.2999999999999998 3.3999999999999999






test_model

```
Note the `` inside `` which is invalid.