ENH: Add functions to calculate the pdf and cdf

[zoj613]

closes #54 and continues #56

In [6]: from polyagamma import polyagamma_pdf
   ...: import matplotlib.pyplot as plt
   ...: import numpy as np
   ...: plt.style.use("seaborn")
   ...: x = np.linspace(0.01, 3, 1000);x.sort()
   ...: for h, z in zip([1, 4, 7, 10, 15, 25], [10] * 6):
   ...:    a = polyagamma_pdf(x, h=h, z=z)
   ...:    plt.plot(x, a, label=f'PG({h}, {z})')
   ...: plt.xlabel('x')
   ...: plt.ylabel('f(x)')
   ...: plt.ylim(bottom=0)
   ...: plt.legend()
   ...: plt.show()

pdf's

cdf's

[codecov-io]

Codecov Report

Merging #57 (22d9ef8) into main (2af0edb) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #57   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           98       122   +24     
=========================================
+ Hits            98       122   +24     

Impacted Files	Coverage Δ
polyagamma/__init__.py	100.00% <100.00%> (ø)
polyagamma/_polyagamma.pyx	100.00% <100.00%> (ø)

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[zoj613]

#58 improves on this by implementing these directly without using slow numerical integration. so closing this.