Protected _headerId in STEPfile (cleditor)

_headerId is a counter used in STEPfile class. Its value can change in ReadHeader and ReadExchangeFile methods. Hence a mutex was introduced in order to protect it.
GetApplication_instanceFromFileId in InstMgr class

The SDAI_Application_instance * GetApplication_instance( MgrNode * ) and MgrNode * FindFileId( int ) methods of class InstMgr were often being used together in the class STEPfile. Hence a special function was created in class InstMgr of the form SDAI_Application_instance * GetApplication_instanceFromFileId( int ) which would take care of locking and consistency issues. This function was then used in STEPfile.
Safe InstMgr iteration in STEPfile

Variables of class InstMgr were being iterated upon in class STEPfile through the variables _instances (including its get function instances()) and _headerInstances. The mtx defined in InstMgr was invoked to make them safe.
Safe attributes iteration in STEPfile
Added a comment about thread safety in STEPfile