Add layout-restricting parameters to BaseDevice

[HGSilveri]

• Adds three new layout-specific parameters to BaseDevice:
  • optimal_layout_filling: An optional value for the fraction of a layout that should be filled with atoms.
  • min_layout_traps: The minimum number of traps a layout can have.
  • max_layout_traps: An optional value for the maximum number of traps a layout can have.
• Includes these new values in BaseDevice.validate_layout()

Closes #749 .