opesci/devito: Devito-3.1

This release includes several new pieces of functionality and some significant API changes: Updated to Python-3 (exclusively) and SymPy-1.1 Renamed symbol types: DenseData->Function, TimeData->TimeFunction, PointData->SparseFunction Introduction of a Grid class that provides the defaulting behaviour for space and time dimensions Enable completely generic operators without explicit substitution of spacing variables Addition of explicit start/end values for dimension iterations for operator invocation Removed fixed dimensions with hard-coded loop bounds Improvements to YASK backend resulting in functional acoustic operators (both forward and adjoint) Addition of new seismic example notebook using Dask for shot-level parallelism Minor changes: Now using nbval to test notebooks Separating Function and Symbol types in internal hierarchy Make Constant a Symbol, not Function Added ability to switch backends at runtime Lots of internal refactoring..