global-workflow YAML templates (pygw)

[guillaumevernieres]

There is probably a better place to document the switch to pygw, but good enough for now.

Here's a copy/paste of @CoryMartin-NOAA 's PR description:

The pygw YAML parser follows this convention:
!INC to include a YAML file
!ENV to denote a environment variable is somewhere in that entry
${VAR} is used for environment variables
$(VAR) is used for variables defined in a python dictionary
{{VAR}} is used for variables defined in a python dictionary that are cycle dependent (note this may need to be in strings to work properly i.e. '{{VAR}}'