Class reorganization and rand_soc

[jgoeders]

This PR:

• Adds rand_soc tool
• Refactor Flow, Tool, run.py, as well as several flows and tools to:
  • Make use of new helper functions added in #316
  • Support passing arguments to Flow
  • Remove redundant code
  • Support Tools that don't get passed a design
  • Allow for a flow to specify a post-execute script (previously there was one hard-coded for the error injector)

Also fixes #347