Incorporation of functionality from GridInterpolations.jl

[GridInterpolations.jl](https://github.com/sisl/GridInterpolations.jl) provides multivariate interpolation on a rectilinear grid with support for both multilinear and simplex interpolation. My lab does interpolation in an inner-inner loop for a dynamic programming application, and so quite a bit of effort was put into making it fast. It would be neat if this functionality can be migrated to Interpolations.jl without compromising on speed or the simplicity of the interface.