Add a more explicit a way to update the origin in TensorMeshes

[Santiago Soler]

In some cases it's desirable to define a TensorMesh that spans through a very specific region but that it also has some padding around it. For example, if a given survey is defined on a (300, 1500, -400, 600) region (defined in West, East, South, North) one might want to define a TensorMesh below it with fine cells inside that region, but with a couple of larger cells around it (the padding). To do so, one need to pass a h parameter like this one:

hx = [
    (d_east, n_pad_east, -stretch_factor),
    (d_east, n_east),
    (d_east, n_pad_east, stretch_factor),
]
hy = [
    (d_north, n_pad_north, -stretch_factor),
    (d_north, n_north),
    (d_north, n_pad_north, stretch_factor),
]
hz = [(d_z, n_z)]
h = [hx, hy, hz]

The issue is that it's not trivial to set what the origin of the mesh should be before actually building it. A way to solve this problem is to create the mesh with a default origin of (0, 0, 0) and then change the origin property. Nevertheless, editing the origin parameter is not explicitly stated on the documentation.

A possible solution to this would be to add a TensorMesh.set_origin() method that sets the origin to a new given value. This method shouldn't be adding any new functionality, but exposing a clear interface on how the origin property could be changed.

I would gladly implement it if this is something that you might want to have in discretize.

Related to ##21

[Joseph Capriotti]

The documentation doesn't make it transparent that you can change the origin of any of the BaseRegularMesh. I would personally be in favor of elevating this availability in the documentation with examples in the origin docstring, and also in the TensorMesh example in its docstring.