Fix multiply defined Error symbols in vtkh_core and vtkh_filters DLLs

Mark inline methods with explicit 'inline' keyword to prevent symbol duplication
when both vtkh_core.dll and vtkh_filters.dll export the Error class.

This resolves linker error LNK2005 for Error constructor and other inline methods
that were being defined in multiple DLLs when both were built with ASCENT_EXPORTS_FLAG.

The explicit 'inline' keyword ensures these symbols are properly inlined at compile time
and not duplicated across translation units in different DLLs.

Fixes Windows build failure: fatal error LNK1169 in vtkh_filters.vcxproj

Author: nicolemarsaglia

src/libs/vtkh/Error.hpp

@@ -14,9 +14,9 @@ class VTKH_API Error : public std::exception
   std::string m_message;
   Error() {}
 public:
-  Error(const std::string message) : m_message(message) {}
-  const std::string & GetMessage() const { return this->m_message; }
-  const char * what() const noexcept override { return m_message.c_str(); }
+  inline Error(const std::string message) : m_message(message) {}
+  inline const std::string & GetMessage() const { return this->m_message; }
+  inline const char * what() const noexcept override { return m_message.c_str(); }
 
 };