Add function to create a tesseroid layer

[aguspesce]

Add function that creates a tesseroid layer, similar to the prism layer. Create
an Xarray DataAccesor that makes available some methods to deal with the prism
layer, included one to compute the gravitational field generated by the whole
layer in a set of computation points. Add checks to avoid generating layers
with tesseroids that overlap around the earth. Add tests and a new gallery
example.

Fixes #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

[santisoler]

Thanks for writing this @aguspesce! I left some minor suggestions. Check them out and don't hesitate to ask any question.

[aguspesce]

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

[santisoler]

Thanks @aguspesce! A minor suggestions here. I'm committing them.