Respect MJCF path processing and XML include specifications

From the MuJoCo docs:

> The full path to a file is determined as follows. If the strippath attribute described above is "true", all path information from the file name is removed. The following checks are then applied in order: (1) if the file name contains an absolute path, it is used without further changes; (2) if this attribute is set and contains an absolute path, the full path is the string given here appended with the file name; (3) the full path is the path to the main MJCF model file, appended with the value of this attribute if specified, appended with the file name.

This is applies to included files, which must be treated as if they were part of the document including them. `dm_control.mjcf`, however, ignores this and treats all paths _relative to the file(s) being included_.

Take the following example:

/root/foo.xml
```xml

...

```
/root/common/skybox.xml
```xml

...
/>

```

This example (were it filled out entirely) compiles successfully using MuJoCo, but fails when using `dm_control.mjcf` which looks for `right.png` in `/root/common/skyboxes/`. It appears the root problem is in `parser.py` which doesn't propagate the the fact that `skybox.xml` was included.

This means the MJCF models in dm_control are incompatible with other MuJoCo-dependent libraries and vice-versa.