Wrapping the blockmedian function!

Description of proposed changes

Current implementation can take a pandas.DataFrame as input and produce a pandas.DataFrame output filtered accordingly.

import pygmt
dataframe = pygmt.datasets.load_sample_bathymetry()
output = pygmt.blockmedian(table=dataframe, spacing="5m", region=[245, 255, 20, 30])
print(f"Original: {len(dataframe)}, Filtered: {len(output)}")
print(output.head())

Original: 82970, Filtered: 5849
   longitude  latitude  bathymetry
0  245.88819  29.97895      -385.0
1  246.96821  29.97529       -96.0
2  245.87456  29.92130      -426.0
3  245.94120  29.89000      -430.0
4  246.95780  29.92900       -96.0

Fixes #348

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 adding new functionality, add an example to docstrings or tutorials.