Add section about cloning conda envs (especially useful when using GPUs)

It will be nice to have another section here, https://hcc.unl.edu/docs/guides/running_applications/using_anaconda_package_manager, about how to create a custom conda environment that utilizes our GPUs, e.g. "Creating an Anaconda Environment that utilizes HCC's GPUs". The main steps are: module load tensorflow-gpu/py36/1.12 conda create -n rl_tensorflow-1.12 --clone $CONDA_DEFAULT_ENV module purge