Function for plotting pdf

[Naokikiki]

Is your feature request related to a problem? Please describe.

plot_pdf_1D()、plot_pdf_2D() : These functions for plotting exact pdf should be implemented in the package. Now users have to define these functions themselves.

Describe the solution you'd like

When you are given pdf explicitly, show the distribution visually especially in 1D or 2D cases. It would be nice if we can show graphs even when we don't know the normalizing constant.