Mallocate memory when making source buffers for scenekit sources.

[dmsurti]

When we import large models as reported in GH Issue #56 and #58,
allocating memory for the buffers on the stack won't work, so we
explicilty allocate it on the heap and free this memory once the
scenekit sources have been generated from these source buffers.

Fixes for #56, #58.

[codecov-io]

Codecov Report

Merging #59 into master will decrease coverage by 1.46%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage   69.13%   67.66%   -1.47%     
==========================================
  Files          14       14              
  Lines        1419     1429      +10     
  Branches      125      127       +2     
==========================================
- Hits          981      967      -14     
- Misses        381      399      +18     
- Partials       57       63       +6

Impacted Files	Coverage Δ
AssimpKit/Code/Model/Tests/AssimpImporterTests.m	59.12% <100%> (-3.58%)	⬇️
AssimpKit/Code/Model/Tests/SCNSceneTests.m	81.81% <100%> (ø)	⬆️
AssimpKit/Code/Model/AssimpImporter.m	85.96% <100%> (-0.62%)	⬇️
AssimpKit/Code/Model/SCNNode+AssimpImport.m	0% <0%> (ø)	⬆️
AssimpKit/Code/Model/SCNScene+AssimpImport.m	70% <0%> (+1.57%)	⬆️

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