First pass implementation for terrain_types folder contents

[Zonr0]

Provide first pass implementation for the terrain_types folder.

First pass implementation means that all (valid) keys used in vanilla files either:

• Have full working implementations OR
• Have working placeholder implementations (eg. simple scalars) with status documented on the tracker OR
• Have false positives in a placeholder or incomplete implementation that properly covers the majority of use cases. OR
• Have false positives or are unimplementable due to bugs or missing features in cwtools core

Additionally, before this issue can be closed, please make sure of the following:

• All significant TODO items are documented in the issue tracker
• Documentation strings are either present or documented as missing in the tracker

That being said, these are just the criteria for this issue and scope item to be considered finished. In these early days, every new definition is incredibly helpful for end users trying to use cwtools! PRs with partial implementations are more than welcome!

This is an auto-generated issue. It may make sense to close this ticket, combine it with others, or split it into smaller chunks.