Function to merge multiple meshes (and their attributes)

[BrunoLevy]

Write a new function to merge multiple meshes and optionally copy their attributes (see #214 moved to Q&A)

[tksuoran]

Just a quick note, somewhat related but possibly separate issue: I have also a need to apply mat4 transformation to attributes when doing the merge. While currently I apply the transformation separately, it might be useful addition to Geogram itself. In order to be able to transform attributes, information on how to apply transformation would be needed per attribute:

• Attribute transforms like position (point) vector
• Attribute transform like normal
• Attribute transform like tangent/bitangent
• Attribute is not affected by transform (most other attributes, such as color, tex coord)