Add better test functions

[gomezzz]

Feature

Desired Behavior / Functionality

Current test functions are based only on polynomials and sinusoidal and exponential functions.

What Needs to Be Done

• [ ] Find nice test functions (e.g. in VEGAS Enhanced paper )
• [ ] Add them to integration_test_functions.py
• [ ] Add them in integration_test_utils.py so they become part of the standard test sweet
• [ ] If necessary adjust current tests in case they fail

How Can It Be Tested

• Run pytest