Data kernel matrix for 2D meshes

This release includes data kernel matrix (method get_straight_rays) for 2D triangular meshes.

New module rgrid

Module rgrid supersedes modules cgrid2d and cgrid3d for increased flexibility in the choice of raytracing algorithm. It also removes a C++ layer that was used to wrap the base C++ code to python.

ttcr with DSPM on 3D rectilinear grids

DSPM implemented on 3D rectilinear grids

ttcr with DSPM

Dynamic shortest path method (DSPM) on 3D unstructured meshes added to the suite of methods

preliminary release

v0.1-beta

bug fix