Trestle automatically creates required directories (e.g. system-security-plan) if absent

[butler54]

Issue description / feature objectives

The opinionated directoires of trestle are 'bulky' for most users. Typically they are using a subset of the directories
(e.g. just the catalog directory, or catalog and profile directories).

As a result users typically want trestle to be able to run without all the directories.

trestle init -gd creates a bare directory. When using this structure effectively we need to guarantee directories are automatically created.

e.g. trestle create --type system-security-plan -o my-ssp

or trestle import --file ../foo.yaml --output stuff wokrs

Completion Criteria

• All file creation functions for oscal objects will automatically create required directories.

[butler54]

@butler54 to write up.

[butler54]

• Doing a little testing this may work already in which case it's a documentation issue.
• trestle create --type system-security-plan -o my-ssp does work.
• We'd need to add testing and update documentation as appropriate.