Misc. typos

Some are doxy comments, some are just trivial source comment typos. 
Found using `codespell -q 3 --skip="./contrib" -I ../assimp-whitelist.txt`
whereby whitelist contained:
```
childs
iff
lod
nto
ot
whitespaces
```

code/ObjExporter.cpp

@@ -155,7 +155,7 @@ std::string ObjExporter :: GetMaterialLibName()
 
 // ------------------------------------------------------------------------------------------------
 std::string ObjExporter::GetMaterialLibFileName() {
-    // Remove existing .obj file extention so that the final material file name will be fileName.mtl and not fileName.obj.mtl
+    // Remove existing .obj file extension so that the final material file name will be fileName.mtl and not fileName.obj.mtl
     size_t lastdot = filename.find_last_of('.');
     if (lastdot != std::string::npos)
         return filename.substr(0, lastdot) + MaterialExt;