Add: handle string attributes

[eprana]

To resolve this issue : #7

String attributes are stored in json file as follow : [ "size",1, "storage","int32", "strings",["/pCube1/pCubeShape1","/pSphere1/pSphereShape1"], "indices",[ "size",1, "storage","int32", "arrays",[[0,1,0,1,0,1,0,1,0,1]] ] ]

Store "arrays" in new 'stringIdxs' attribute

[eprana]

@aoblet & @AlexiaLT if you want to have a look.