Merge branch 'virtualenv' into 'master'

Add requirements.txt file for creating virtual env for running Prefactor

See merge request eosc/prefactor3-cwl!61

build_venv.sh

+#!/bin/sh
+#
+# Build a virtual environment that can be used to run the Prefactor pipeline
+
+python3 -m venv venv
+. venv/bin/activate
+
+python3 -m pip install --upgrade pip
+python3 -m pip install --upgrade wheel
+
+python3 -m pip install -r requirements.txt
+

requirements.txt

+toil @ git+https://github.com/DataBiosphere/toil.git@27dae4cb1ea7e9df56c5e901ec786387c8111f5b#egg=toil[cwl]