Version 1.3.0: Triangle Sets and Mirroring

This is a minor release of the Core Specification of 3MF.

It clarifies several ambiguities, resolves inconsistencies and adds new concepts.
New Concepts compared to 1.3.0:

• trianglesets allow to encode sets of triangles into 3MF-files. Applications can apply grouping operations and assign properties to a list of triangles.
  Figure: Triangle sets that correspond to engineering surfaces of a part. One of the triangesets is rendered differently to indicate selection.

• Mirroring of Meshes: the mirrormesh-element allows to explicitly encode the relation of one mesh-object to be a mirrored instance of a different mesh in the 3MF file.
  Figure: a mesh, the mirror plane and its mirrored copy
  

• reccomendedextensions is a list of extension namespace prefixes that allow to encode which extensions are recommended for a consumer to read a file.

Clarifications compared to v1.2.3:

• the interpretation of transformations with negative determinants has been clarified
• the usage of the ZIP-format and the zip64 extensions has been clarified
• the notion of object-level and package-level thumbnails has been clarified