PyMJCF nested include tags relative to base model

[guyazran]

Addresses #418

[jonzamora]

Hi @guyazran , did you happen to test this out before making your Pull Request here?

e.g. on the example you provided here: https://github.com/google-deepmind/dm_control/issues/418

I am also interested in composing XML files with PyMJCF which support nested include tags, and i'm hoping to revive this issue so things can be resolved for us and others.

[guyazran]

Hi @jonzamora. Thanks for reviving this PR. It will make running with nested include tags much smoother.

I sent this PR in a long time ago, but I can definitely say that at the time of creating the PR and when I wrote "addresses #418" the example scene in that issue ran smoothly.

However, this might be a pointless effort because in another issue the creators said that they are working on a different solution #407. perhaps you should ping there and see how the alternative solution is working