Staggered Support/Updated UI

Major reworkings to several aspects of Devitoboundary operation to improve reliability and allow for staggered grid support:

Surfaces and distance:

• Axial distance calculation simplified, with some calculations moved outside operator
• Distances are now treated as matched pairs of boundary-adjacent points when evaluating stencils

Stencils:

• Stencil evaluation now carried out using Pandas
• Offset stencils for staggered grids now supported
• Stencils can now be accessed as an array of functions corresponding to each coefficient

UI:

• ImmersedBoundary interface reworked to use Pandas DataFrames for specifying functions and respective boundary conditions
• Pandas DataFrames used for specifying derivatives

Tests:

• Additional tests including a benchmark against a conventional free-surface boundary

v0.1

Initial release of Devitoboundary with basic immersed boundary support for non-staggered grids in Devito. API likely to be subject to change in forthcoming versions. Includes an example topography implementation with the 3D acoustic wave equation.