Figure.histogram: Add the parameter 'weights' for weighting data

[seisman]

In histogram, -Z+w means using the second data column as weights. This PR provides the weights parameter which is more Pythonic. It can cover three use cases:

1. data is a file: weights=True means using the 2nd column as weights
2. data is a 2-D array: weights=True means using the 2nd column as weights
3. data is a 1-D array: weights must be a 1-D array as weights

For comparison, matplotlib.pyplot.hist also provides the weights parameter.

Here is an example showing that PyGMT and matplotlib produce the same plot:

data = [1, 1, 1, 1, 1, 1, 2, 2, 2, 3, 4, 5, 6, 7, 8, 8, 8, 8, 8, 8]
weights = [0.1, 0.1, 0.2, 0.2, 0.3, 0.3, 0.4, 0.4, 0.5, 0.5, 0.6, 0.6, 0.7, 0.7, 0.8, 0.8, 0.9, 0.9, 1.0, 1.0]

import matplotlib.pyplot as plt
import numpy as np

plt.hist(data, weights=weights, bins=np.arange(0, 10, 1))
plt.show()


import pygmt
fig = pygmt.Figure()
fig.histogram(data=data, weights=weights, series=1, fill="lightblue", frame=True, region=[0, 10, 0, 0])
fig.show()