Remove depth_type from EquivalentSources

[santisoler]

Remove the depth_type argument from the EquivalentSources constructor. By default the point sources will be defined using a relative depth strategy. Removed the deprecated relative_depth argument from the constructor. Update the tests for the class and its methods.

Todo:

• remove depth_type from gradient boosted equivalent sources and update their tests
• remove depth_type from the docs (if any)
• check if we need to update the spherical equivalent sources as well

Relevant issues/PRs:

Fixes #423

[santisoler]

Co-authored by @indiauppal as well during a Fatiando Meeting in which we all worked together on this code.