Add libSBML importer and optional GTK4 GUI; refactor build and introduce progress callback; fix delay-structure bugs

[rafaelstjf]

Motivation

• Enable importing SBML/XML models and provide a GNOME/GTK frontend while keeping the simulation core reusable.
• Expose runtime progress so a GUI can plot simulation state as it runs.
• Improve robustness and memory handling in delay structure implementations and in the rejection method.
• Modernize the CMake layout to produce a reusable stocbionet_core library and conditionally enable libSBML and GTK/libadwaita support.

Description

• Refactor build system in CMakeLists.txt to create stocbionet_core, link Threads::Threads, and optionally detect and link libSBML and GTK/libadwaita to build StocbionetGui, with informative configure messages.
• Add SBML importer (src/sbml/SBMLImporter.hpp and src/sbml/SBMLImporter.cpp) that conditionally uses libSBML to translate SBML models to the internal STOCBIONET format and add Model integration via SBMLImporter::canLoad/load.
• Add a GTK4/libadwaita GUI (src/gui/main_gui.cpp) that uses a background thread to run simulations and receives runtime updates via a new progress callback.
• Introduce a progress callback API: add setProgressCallback to Log and SSA, store a function<void(long double,const double*,int)> progressCallback, and call it from Log::insertNode so external code (GUI) can receive streaming data.
• Fix and harden delay-structure code and rejection method: add RejectionMethod constructor and destructor handling, defensively free arrays with delete[], ensure default chooseStructure case, and fix bugs/edge cases and resizing logic in CircularList, DelayList, DelayHeap, and DelayHash.
• Update README.md with documentation for SBML import support and the optional GTK GUI.

Testing

• Performed a CMake configure and full build (producing stocbionet_core and Stocbionet) on a Linux development environment with libSBML and GTK4/libadwaita available and the build completed successfully.
• Verified that SBML files are detected by SBMLImporter::canLoad and that Model::loadModel uses the importer path when applicable, with basic translation of parameters, species and reactions.
• No automated unit tests are present in the repository; no automated test suite was executed as part of this change.

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Codex Task