Add Singularity build recipe and run script.

[prehensilecode]

Added a Singularity build recipe, and a run_singularity.py script similar to the existing run_docker.py script. The run_singularity.py script takes the same options/flags as run_docker.py

run_singularity.py was tested with both the following (Singularity 3.8.1 + Python 3.9.1 + absl-py 0.13.0 on RHEL 8.1 with Nvidia Tesla V100-SXM2):

    python3 ./singularity/run_singularity.py --fasta_paths=T1050.fasta --max_template_date=2020-05-14
    python3 ./singularity/run_singularity.py --fasta_paths=T1050.fasta --max_template_date=2020-05-14 --preset reduced_dbs

[Augustin-Zidek]

Unfortunately we don't have the capacity to maintain and support Singularity. We added a link in the README to the two AlphaFold issues that have links to third party repositories with Singularity definition files (#10 and #24).