Added DynamicArray class for dynamic buffer data.
Added method to register external initializers.

* Modules which create Mesh objects can register functions using the
register_initializer function. For example, extra.PLY.utils adds the
function from_ply which is more consistent with existing methods
(Mesh.from_gridded_data, Mesh.from_ply, etc.).
Added Solver class for dynamic systems.
Merge branch 'dynamic-buffer' into dynamics