Fix GSPH Python bindings not being compiled

Add pyGSPHModel.cpp to CMakeLists.txt Sources list. The file was
added in PR Shamrock-code#1453 but was missing from the build configuration,
causing shamrock.get_Model_GSPH() to not be available at runtime.

Author: Guo-astro

src/shammodels/gsph/CMakeLists.txt

@@ -11,11 +11,12 @@ cmake_minimum_required(VERSION 3.9)
 
 project(Shammodels_gsph CXX C)
 
-# Sources: GSPH solver, Model, and VTK I/O
+# Sources: GSPH solver, Model, VTK I/O, and Python bindings
 set(Sources
     src/SolverConfig.cpp
     src/Solver.cpp
     src/Model.cpp
+    src/pyGSPHModel.cpp
     src/modules/UpdateDerivs.cpp
     src/modules/io/VTKDump.cpp
 )