Add installation deps object

[njtierney]

• Added option to restart R + run library(greta) after installation (#523)
• Added installation deps object, greta_python_deps() to help simplify specifying package versions (#664)
• removed method and conda arguments from install_greta_deps() as they were not used.
• removed manual option in install_greta_deps()
• added default 5 minute timer to installation processes

Resolves #664