[feature] Add depth_interp method

[oj-tooth]

Add a new depth interpolation method to NEMODataArray and NEMODataTree to linearly interpolate along 1- and 3-dimensional depth coordinates.

To be compatible with both depth{t/u/v/f/w}(k) and depth{t/u/v/f/w}(k, j, i)

• Add functions to compute interpolation weights and apply interpolation along the depth coordinate.
• Add new depth_interp method to NEMODataTree class.
• Add new depth_interp method to NEMODataArray class.

Relevant Modules:

• nemo_cookbook/nemodatatree.py
• nemo_cookbook/nemodataarray
• tests/test_nemodatatree_core.py
• tests/test_nemodataarray_core.py
• tests/test_interpolate.py