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 )

src/pystencilssfg/cmake/FindPystencilsSfg.cmake

@@ -48,7 +48,7 @@ endif()
 if(${PystencilsSfg_FOUND})
     message( STATUS "Found pystencils Source File Generator (Version ${PystencilsSfg_VERSION})")
     message( STATUS "Using Python interpreter ${PystencilsSfg_PYTHON_INTERPRETER} for SFG generator scripts.")
-    
+
     execute_process(COMMAND ${PystencilsSfg_PYTHON_INTERPRETER} -m pystencilssfg cmake modulepath --no-newline
                     OUTPUT_VARIABLE _PystencilsSfg_CMAKE_MODULE_PATH)