Provide back compatible vgdml frontend interfaces

!805 (merged) introduced a potentially breaking interface change to vgdml::Frontend::Load with it returning a unique_ptr<Middleware> instead of bool. No change in behaviour results if the call is made in a conditional or the return value stored in an auto variable only used in a conditional. Some clients may store the return value in an explicitly typed bool variable, leading to compile errors.

Reinstate original bool Frontend::Load function to ensure compatibility for older clients.

Implement trivial Parser struct with Load member function returning the Middleware unique_ptr and same signature as free Load function. This is implemented as a struct+member function rather than namespace+free function or a static member function as future updates may store additional state or provide more interfaces for querying the parsed GDML.