WIP Add function to create a tesseroid layer

[aguspesce]

Fix #83

[aguspesce]

Hello @leouieda and @santisoler I have a little problem with this PR, so I need your wisdom!

I coded the function to calculate a teseroid layer and its gravity. I also created an example using Esaio's topography. When I ran the example, it calculates the teseroid layer, and I think it's done well. But when it calculates the gravity, it returns this error:

 $ python -i examples/forward/tesseroid_layer.py
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/agustina/Github/harmonica/examples/forward/tesseroid_layer.py", line 41, in <module>
    gravity = tesseroids.tesseroid_layer.gravity(grid_coords, field="g_z")
  File "/home/agustina/Github/harmonica/harmonica/forward/tesseroid_layer.py", line 269, in gravity
    return tesseroid_gravity(
  File "/home/agustina/Github/harmonica/harmonica/forward/tesseroid.py", line 174, in tesseroid_gravity
    dispatcher(parallel, density)(
SystemError: CPUDispatcher(<function jit_tesseroid_gravity at 0x7f9a26841670>) returned a result with an error set

I know that I have a zero division, but I don't know where is the problem... It is my code that create wrong the layer or the tesseroid function to calculate the gravity. Please... Ideas, suggestions, something?

[santisoler]

Sorry for not replying directly in this PR, @aguspesce. For the record, the zero division error was due to the fact that the forward modelling wasn't ignoring tesseroids with zero volume. You have already fixed it on #339.

How is the status on this PR? Is it ready for review?

[aguspesce]

Hello @santisoler. I've working in the test. If you like, you can review the example and the teseroid_layer function

[aguspesce]

Hello @santisoler. I think that this PR is ready for your review

[aguspesce]

@santisoler... when you have some time, take a look at the code