Material mapping and assignment

[ClemensLinnhoff]

Describe your changes

• Put material mapping in separate file.
• Add texture assignment to asset file.
• Restructure documentation to contain the different files.

Check out the implemented documentation on my fork:
https://clemenslinnhoff.github.io/OpenMATERIAL/asamopenmaterial/latest/specification/index.html

Issue ticket number and link

Fixes #46, fixes #51, fixes #109

Checklist before requesting a review

• I have performed a self-review of my code/documentation.
• My changes generate no new warnings during the documentation generation.

[AsamDiegoSanchez]

Reviewed
Signed-off by: Diego Sánchez [email protected]

[ClemensLinnhoff]

@LudwigFriedmann could you also review this, if the new structure would work for you? This also contains the decision to have a material texture assignment in the asset file and the material mapping in a separate mapping file.
The description of the general file structure however is not implemented here, yet.

[ClemensLinnhoff]

As soon as this is merged I will create an example to showcase both the mapping via texture and via mesh.

[LudwigFriedmann]

LGTM