STEPattribute member _derive: public, int -> protected, bool
check for redefine in attr being copied _from_, not to - ?!

I do not understand how this could have been right or how it did not
cause problems if wrong. Perhaps redefined attr's are used so
infrequently that it went unnoticed?
move clstepcore cpp test dir into clstepcore
skeleton for the operators/constructors tests for STEPattribute
improve stepattr copy constructor

uses shallow copy, so access violations are possible
test_inverse_attrs2 works now because of the STEPattr copy ctor fix