Parallel tesseroid computation doesn't work with generators

[leouieda]

Parallel execution of fatiando.gravmag.tesseroid functions doesn't work on generators. For example:

def modelgen(lat, lon, topo, step):
    for i in range(topo.size):
        t = Tesseroid(w=lon[i] - step/2,
                      e=lon[i] + step/2,
                      s=lat[i] - step/2,
                      n=lat[i] + step/2,
                      top=topo[i],
                      bottom=0,
                      props={'density': 2670})
        yield t

model = modelgen(lat, lon, topo, step)
gz = tesseroid.gz(lat, lon, height, model, njobs=8)

will fail because multiprocessing can't pickle the generator object.

A possible way around this is to use a better multiprocessing alternative (toolz, joblib, dask).

[eusoubrasileiro]

just following your effort on using multiprocessing instead of cython