Because it was historically grown, we have some duplicated code for
converting script types to native types.
This patch removes the file JSConversions.cpp and moves some code to
ScriptConversions.cpp.
The places using JSConversions.cpp are changed to use the
ScriptInterface's conversion functions in ScriptConversions.cpp.
I also removed JSInterface_Vector3D because it had additional
requirements to the conversion code that no other code has and because
it's currently not used. I think it doesn't make sense to maintain code
just because it could possibly be used again later.
Closes#2213
Refs #1886
This was SVN commit r14036.
