Addresses #2813
get_dependencies() was pair coded with @eb8680

This adds a helper get_dependencies() and uses that to automatically create a sparse dependency graph in the AutoStructured guide.

Tasks

• Implement a get_dependencies() helper
• Integrate automatic guide structure in AutoStructured
• Convert prior structure to posterior structure as in (Webb et al. 2018)

Tested

• tests for get_dependencies()
• smoke tests for AutoStructured with default params
• test of posterior guide structure