Devito-3.0.3

A utility release to ensure a stable version before a set of planned API changes. Includes changes and some signficant progress on:

• Prototype YASK-backend
• Improved arguments handling
• Seismic examples and tutorials
  • RickerSource and demo_model utilities
• New TTI formulation (shifted and centred)
• Anaconda install
• Autotuning fixes
• Compiler-debug model