Add inline example for grdvolume

[willschlitzer]

This PR adds an inline code example to the docstring for grdvolume.

Addresses #1686

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

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[seisman]

Looks good to me except a typo.

[seisman]

Need to fix the styling issue before merge.

[weiji14]

Ideally, we would have nice column names like in grdhisteq:

pygmt/pygmt/src/grdhisteq.py

Lines 301 to 309 in d85bfdf

	>>> bins = pygmt.grdhisteq.compute_bins(grid=grid, divisions=5)
	>>> print(bins)
	start stop
	bin_id
	0 179.0 397.5
	1 397.5 475.5
	2 475.5 573.5
	3 573.5 710.5
	4 710.5 2103.0

But for the sake of time, and just finish this inline example, let's go with this:

Suggested change

	"""
	>>> print(output_dataframe)
	0 1 2 3
	0 200 2.144285e+12 7.972228e+14 371.789489
	1 250 2.104042e+12 6.908183e+14 328.329232
	2 300 2.014978e+12 5.877195e+14 291.675420
	3 350 1.892109e+12 4.897545e+14 258.840510
	4 400 1.744792e+12 3.988316e+14 228.584026
	"""