Allow Singularity to run containers

[jpdna]

So far the -use_singularity option has only been added for BWA and tested successfully, but that should serve as a model for the other Cannoli commands.
I will implement and test the other commands over the next days.

Overview:
Singularity is a containerized execution environment similar to Docker, and can directly use Docker images.
It is currently common for HPC cluster administrators to forbid the direct use of Docker due to security concerns, but often do support Singularity, making it the only containerized option available to these users.

The primary difference in parameters to Singularity have to do with the volume mapping into the container. The -B bind option in Singularity works similarly to the -v option in Docker, however I was not able to map the same arbitrary path as Docker does in

 "-v", "%s:%s".format(mountpoint, mountpoint),

However I was able to map an external path to the within container location /mnt/ as in:

"%s:%s".format(mountpoint, "/mnt/"),

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/cannoli-prb/105/
Test PASSed.

[coveralls]

Coverage increased (+0.2%) to 31.773% when pulling 9e5880f on jpdna:singularity_v2 into 276be85 on bigdatagenomics:master.

[heuermh]

I will implement and test the other commands over the next days.

I'd hold off on that for now, #69 should be merged first or the diffs will be hard to manage.

[heuermh]

I haven't gone through the Singularity install on my Mac yet (will soon) and the web docs aren't that helpful, what is -q for?

[heuermh]

Don't see -q listed at https://github.com/singularityware/singularity/blob/master/libexec/cli/exec.info

[heuermh]

is there a --long-argument form for -B?

[heuermh]

Yes, --bind

[heuermh]

Does Singularity support --rm, or similar flag to remove the image from disk when complete?

[heuermh]

No, it does this by default

[heuermh]

in Nextflow it looks like they use -B mountpoint (without the target) or -B mountpoint:mountpoint:ro to mount read-only, might this work?

[heuermh]

@jpdna @fnothaft @akmorrow13 Please take a look at WIP #107, that is the direction I'm currently headed.

Usage would look something like

val builder = CommandBuilders.create(args.useDocker, args.useSingularity)
  .addEnvironment("ENV", "value")
  .addMount("/source", "/target")
  .addFile("/file/to/add")
  .setTimeout(1000, TimeUnit.SECONDS)
  .setSudo(true)
  .setImage("namespace/image")
  .setExecutable("bwa mem")
  .addArgument("-t", "1")
  .addArgument("-")

genomicRdd.pipe(
  builder.build(),
  builder.getEnvironment(),
  builder.getFiles(),
  builder.getOptTimeout(),
  builder.getOptFlankSize())

[heuermh]

Closing in favor of #107.