Refactor schema_reader.py

[webmat]

This is a small cleanup to clarify schema_reader.py. It would need more <3 but
that's a start.

In this PR:

• No changes to the generated artifacts, except for the removal of the order attribute.
  This attribute was initially meant to capture and reuse the order in which
  fields are defined in the source yaml file. This was never used. Now it's gone.
• Reorder the code in schema_reader.py to make it easier to follow the logic.
• Move some helper functions to ecs_helpers.py
• Add a few doc strings
• Replace a helper function by proper use of Python, which I learned from @marshallmain's contributions. Thanks Marshall :-)

I'll continue work on #809 on top of this. But I want to separate the work a bit,
since significant underlying improvements to the tooling are still needed, to deliver #809.

[marshallmain]

Looks good to me except the ecs_flat.yml and ecs_nested.yml files got deleted, was that intentional?
EDIT: nevermind, the large diff didn't load by default and i read that as file deleted.

LGTM

[webmat]

Whoa you had me scared, there for a second 😂

Thanks