Add a mechanism for building a cache of "system parameters"

- Add a system_parameters module to eryntools that implements some basic types and can write a cache to a file.
- Add a system-parameters Python script that builds the cache.
- Add init_system_parameters to zsh that exports an environment variable that points to the parameters file
- Add an eryn.system-parameters.plist LaunchAgent file for running the script at regular intervals

zshenv

@@ -22,6 +22,7 @@ autoload -Uz do_init_functions
 
 typeset -a zsh_init_env_functions=( \
     init_path \
+    init_system_parameters \
     init_env \
     init_env_aliases \
     init_env_python \