Misc changes for the simulation server

• Add "top level" exports to raps/__init__.py
• Resolve paths more consistently regardless of cwd
  • paths in system and sim configs are resolved relative to the yaml file instead of current cwd
• Add some validation and custom types for system and partitions so that an invalid system raises a validation error immediately instead of when the sim starts
• Floor input dates to time_unit
• Add method on Engine to get all stats
• Add option to stats function to skip some expensive stats calculations
  • The simulation server calls stats frequently to create the timeseries
• Add get_gauge_limits method back
  • We used to have this on the scheduler and the simulation server used it to get the ranges in the dashboard. Add it back as a method on Engine