configurable prepare_th as alternate to custom multi_clip.py scripts

[water-e]

Will take yaml
Should document the sources of data:
1. Capture the git version
2. Yaml input not just a script
3. Users should create dated files, not elapsed
4. Possibly need to move gates/barriers to BayDeltaSCHISM/time_history/structures

Requirements:
1. Should (?) borrow from existing yaml parser with substitution