Deprecate the "test" functions in our packages

[leouieda]

Description:

All our packages include a test function in their base __init__.py that was used as a way to test the version of the package that has actually been installed. But now they are not needed and hardly ever used since the same functionality can be achieved with pytest --pyargs PACKAGE_NAME.

We should deprecate these functions by including a FutureWarning when they are called. The functions would then be removed in the next major release or after 2 minor releases for packages pre-1.0.

See fatiando/verde#344 for a template.

Apply to:

• [x] boule
• [ ] harmonica
• [ ] pooch
• [x] verde

Further instructions:

• Start by opening Pull Requests on each repository listed above.
• Optionally, we can open Issues on each repository if further discussion specific to that repository is needed.
• Mention this Issue on every Issue or Pull Request opened on each opened: Related to fatiando/community#XX
• Check-off the repository on the list above once the Pull Request is merged.
• Close this issue when all items are checked-off.

We want your help!

We know that maintenance tasks are very demanding, so we don't expect a single person to tackle this issue by themselves. Any help is very welcomed, so please comment below that you want to take care of the changes on any repository and we will assign it to you.

[leouieda]

Removing it outright in fatiando/boule#116 since this is such a minor thing that only devs should be using anyway.