Consolidate the different ways of finding depth coordinates

[mx-moth]

In no particular order, there is:

• Convention.depth_coordinate
• Convention.depth_coordinates
• Convention.get_depth_name()
• Convention.get_all_depth_name()
• Convention.get_depth_coordinate_for_data_array()

The Convention.get_depth_name() and Convention.get_all_depth_names() methods seem superfluous. They can be replaced with Convention.depth_coordinate.name or [c.name for c in Convention.depth_coordinates] respectively.

[mx-moth]

Closed by #143