BEAST WDL workflows

[dpark01]

This adds two WDL workflows:

1. beast_gpu -- for running BEAST on a GPU instance. Input is a BEAUTi XML, the output are logs and trees for Tracer/TreeAnnotator/LogCombiner
2. beast_to_auspice -- for taking TreeAnnotator output (a single MCC .tree file) and using augur import beast to prepare auspice-ready files for visualization

This closes #66.

• beast_to_auspice has been tested and a minimal execution test has been added to cromwell-local & miniwdl on Travis
• beast_gpu cannot be tested on a local execution environment or with miniWDL. The WDL is supposedly DNAnexus compatible, however our contract does not yet enable GPU resources, so we cannot test there until we contact sales. It is currently being tested on Terra/GCE under this running job. This code is based on a docker image that has been successfully used by our group several times in the past using dsub / PAPIv2 on GCE, so Terra ought to work--I'm more curious about other platforms (e.g. DNAnexus). Given that testing this workflow on non-Terra platforms is not straightforward for us, I plan to merge this PR as soon as we have confirmation that it works on Terra.

[dpark01]

It works!