Adding option to automatically installation

This extends FindPystencilsSfG.cmake with a Cache Variable CODEGEN_PRIVATE_VENV. This is set by default and if activate it creates a python venv and installs either dependencies from a requirements.txt or if this is not present it downloads the latest pystenicls-sfg/pystencils versions. If a custom Interpreter is passed to cmake this deactivated.

In external CMake Projects this files could by included via

## Pystencils-sfg setups
set(FIND_PYSTENCILSSFG_CMAKE_PATH "${CMAKE_BINARY_DIR}/_pystencilssfg-cmake/")
file(DOWNLOAD
    "https://i10git.cs.fau.de/ob28imeq/pystencils-sfg/-/raw/cmake_setup/src/pystencilssfg/cmake/FindPystencilsSfg.cmake?ref_type=heads"
    "${FIND_PYSTENCILSSFG_CMAKE_PATH}/FindPystencilsSfg.cmake"
)

SET ( CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${FIND_PYSTENCILSSFG_CMAKE_PATH})

find_package( PystencilsSfg REQUIRED )

standalone/CMakeLists.txt

 cmake_minimum_required( VERSION 3.24 )
 project(PystencilsSfg_Standalone)
 
-set(CODEGEN_PRIVATE_VENV ON
-    CACHE BOOL
-    "Create a private virtual Python environment inside the build tree for code generation"
-)
 if (DEFINED CACHE{PystencilsSfg_PYTHON_INTERPRETER})
-	set( CACHE{CODEGEN_PRIVATE_VENV} OFF)
+    set( _use_venv_init OFF)
 elseif(DEFINED PystencilsSfg_PYTHON_PATH)
-	set( CACHE{CODEGEN_PRIVATE_VENV} OFF)
+    set( _use_venv_init OFF)
+else()
+    set( _use_venv_init ON )
 endif()
 
+set(CODEGEN_PRIVATE_VENV ${_use_venv_init}
+    CACHE BOOL
+    "Create a private virtual Python environment inside the build tree for code generation"
+)
+
 function(codegen_venv_install)
     if(NOT CODEGEN_PRIVATE_VENV)
         return()