Implement distributed lensing

[EiffL]

This issue is to track the development of a distributed version of our FlowPM lensing implementation.

We essentially need to create a distributed version of these two funnctions:

• flowpm/flowpm/raytracing.py

  Line 49 in 9145d0c

  def density_plane(state,

• flowpm/flowpm/raytracing.py

  Line 112 in 9145d0c

  def convergenceBorn(cosmo,

The first one should be easily adaptable from the distributed versions of cic_paint which can be found here:

flowpm/flowpm/mesh_utils.py

Line 125 in 9145d0c

def cic_paint(mesh, part, halo_size, weight=None, name=None):

The second one is a tiny bit more sophisticated because it will involve a distributed interpolation operation and this might require some thinking