Added Mathematica notebook with examples of the modified integrals.

[crisely09]

The examples include electron-repulsion integrals:

• Error function
• Gaussian function
• Arbitrary power of r^a, for a<3
• Delta function
• Intracule (delta) integrals and nuclear-electron attraction integrals:
• Error function
• Gaussian function

The Mathematica notebook has one example per type of integral.