Improve Herbie's model templates

[blaylockbk]

The goal of these changes to the model templates is to help improve maintainability in the model templates.

• Do the input validation to the model template rather than in the Herbie class. This will be nice since models sometimes need unique conditions or arguments that are not needed for other models.
• Control the Herbie arguments from the model template, and alert the user when wrong input is given.

General rules for Herbie model templates

1. Each template will represent data from the same NWP model; data sources may be in different locations.
2. The template will include three sections
   1. Metadata
   2. Validation
   3. Build Source URLs
3. Somehow need to keep the template simple enough (and predictable) that others can help keep these updated when needed.