Test for make_system plastic feature

[sannant]

We need to replace LinearizeCore Potential by .linearize_core in make_system.

related to #63

[sannant]

The problem has been fixed. But we need a test, at least testing functionality without bug.

I suggest to test a system with very low or even no adhesion against simulations using Polonsky and Keer.

@sitangshu, since you plan to use the plastic features, are you willing to write a test ?

You can find an introduction to the concept of unit testing in the book
Scopatz and Huff 2015 - Effective Computation in Physics,
but we use the pytest package to implement the tests

An example of test:
test/test_wavy_adhesive.py

and
https://github.com/ContactEngineering/ContactMechanics/blob/master/test/test_plastic.py

Part of the plastic functionalities are already tested low-level in
test_minimization_parabolic_cutoff_linear_core