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/LWSLoader.cpp

@@ -471,7 +471,7 @@ void LWSImporter::BuildGraph(aiNode* nd, LWS::NodeDesc& src, std::vector<Attachm
 // Determine the exact location of a LWO file
 std::string LWSImporter::FindLWOFile(const std::string& in)
 {
-    // insert missing directory seperator if necessary
+    // insert missing directory separator if necessary
     std::string tmp;
     if (in.length() > 3 && in[1] == ':'&& in[2] != '\\' && in[2] != '/')
     {