add GNU parallel (#150)

* add GNU parallel

this will be helpful for running multiple instances of `fire` across the number of cores the container image has access to

* add parallel to list of ubuntu packages

Author: tomeichlersmith

Dockerfile

@@ -23,6 +23,7 @@ RUN install-ubuntu-packages \
     gcc g++ gfortran \
     locales \
     make \
+    parallel \
     wget
 
 # Basic python support, necessary for the build steps.

docs/src/ubuntu-packages.md

@@ -42,6 +42,7 @@ g++ | GNU C++ Compiler
 gfortran | GNU Fortran Compiler
 locales | Configuration of TPython and other python packages
 make | Building system for dependencies and ldmx-sw
+parallel | [GNU Parallel](https://www.gnu.org/software/parallel/man.html) for multi-processing ldmx-sw
 wget | Download source files for dependencies and ldmx-sw Conditions
 python3-dev | ROOT TPython and ldmx-sw configuration system
 python3-numpy | ROOT TPython requires numpy and downstream analysis packages