renaming functions

[isabelizimm]

closes #53

• vetiver_write_docker ➡️ vetiver.write_docker
• vetiver_write_app ➡️ vetiver.write_app
• vetiver_pin_read ➡️ VetiverModel.from_pin()
• internal function vetiver_meta ➡️ _model_meta
• more explicit imports

not included in this PR, to do later

• vetiver_pin_write ➡️ VetiverModel.to_pin()

[juliasilge]

Disregard that last comment I made! Pre-coffee confusion, I think 😬

This looks good to go 👍