Refactor mexximp mex-functions around a stand-alone utility.

[benjamin-heasly]

Currently, Matlab interfaces with Assimp via the in-memory mex function interface. This is an efficient interface.

However, it has become difficult to build, maintain, and distribute mex functions that use shared libraries like Assimp. Two main reasons are:

• On OS X, Matlab's mex build configuration often goes out of date with respect to the OS X/XCode build tools.
• On Linux, Matlab distributes and loads its own version of system libraries like libstdc++, which may be incompatible with the library versions that are required for shared libraries like Assimp (see Incompatibility Between Assimp 3.3.1 and MATLAB #2).

To alleviate these problems, we should decouple mexximp from the Matlab mex function interface. We should make mexximp into a stand-alone utility for reading and writing Assimp models to and from a simple structured representation, like ubjson.

We could continue to use mexximp from Matlab by reading and writing ubjson files or streams, for example with jsonlab. We could choose the schema of the ubjson representation to match the current Matlab struct schema. We could include m-function wrappers with the same behaviors as the current mex-functions, and minimize the impact on existing mexximp code.

As a stand-alone tool, the ubjson version of mexximp would have several advantages:

• easier to build
• available to use for Matlab-based projects
• available to use for non-Matlab-based projects
• able to be distributed as a Docker image