pygmt.grdfill: Add new parameter 'inquire' to inquire the bounds of holes

[seisman]

Implement the idea in #3768 (comment).

Example

>>> import pygmt
>>> import numpy as np
>>> from pygmt.helpers.testing import load_static_earth_relief

>>> grid = load_static_earth_relief()
>>> # One hole
>>> grid[3:6, 3:5] = np.nan
>>> pygmt.grdfill(grid, inquire=True)
array([[-52., -50., -21., -18.]])

>>> # One more hole
>>> grid[7:9, 2:4] = np.nan
>>> pygmt.grdfill(grid, inquire=True)
array([[-53., -51., -17., -15.],
       [-52., -50., -21., -18.]])

[seisman]

I feel this PR is ready for review, and it would be good if we can make it into v0.15.0.