This function checks if the specified Conda environment exists. If it does, the function sets the
RETICULATE_PYTHON environment variable to the path of the Python interpreter in that environment.
If the environment does not exist, the function stops with an error message.
Users need to run this function in every new R session where they want to use your package. Also,
they should run this function before loading your package with library(), because the RETICULATE_PYTHON
environment variable needs to be set before reticulate is loaded.