cmake: added -O2 which gives the performance improvement needed.

src/classes/CMakeLists.txt

@@ -103,6 +103,11 @@ set_target_properties(PUserFcnBase
 target_include_directories(
   PUserFcnBase BEFORE PRIVATE $<BUILD_INTERFACE:${MUSRFIT_INC}>
 )
+
+#--- add compile options for optimization -------------------------------------
+target_compile_options(PMusr PUBLIC -O2)
+target_compile_options(PUserFcnBase PUBLIC -O2)
+
 #--- add OpenMP compile options if needed -------------------------------------
 if (OpenMP_FOUND)
   target_compile_options(PMusr PUBLIC ${OpenMP_CXX_FLAGS})