Fixing memory leak issue

[harris2001]

Just a minor patch to fix the memory leak issue when using ScopePtr in VCDVariable and updating Readme.md

• The original code introduced a circular reference between VarPtr and ScopePtr pointers which was causing memory leak.
• The updated code uses a non-owning weak_ptr to provide temporary ownership for the ScopePtr pointer and deallocate it after it's been used.

P.S. Thanks again for building this project, it's been very useuful!