Update example configs to reduce duplication

[tomeichlersmith]

This will just make the config files more portable and reduce duplication since most folks have all their hps stuff underneath one parent directory.

This can be achieved by providing a [DEFAULT] section at the top of the config file and then using the variable-expansion syntax %(variable_name)s. For example, I do the following:

# define where all my HPS stuff is
[DEFAULT]
hps_home = /full/path/to/hps/working/dir

# can reference that root directory in the rest of the config parameters
[MG4]
madgraph_dir = %(hps_home)s/mc/generators/madgraph4/src

We could also support all environment variables by providing the os.environ map to the ConfigParser constructor as "defaults". Then folks could pull (for example) %(HPS_HOME)s from the environment instead of having to change the config file.

[JeremyMcCormick]

I like the usage of an hps_home var to reduce duplication, though I don't think I would load the full environment into the config parser. Part of the point of using these config files is separating the hps-mc configuration from the OS environment.