PyARC upgrade

Various upgrade in PyARC structure should consider:

• Update tutorial to rely on ABTR concept. Objective: provide more realistic core design that is well referenced, still simple. Verify good performance of ARC codes when compared to OpenMC (for K-eff). Assess impact in different modeling approach ()1/2 step XS processing, Heterogeneous processing, VARIANT).
• Switch PyARC and its suite of unit tests to Python 3 - it should remain compatible with Python 2.
• Upgrade memory data structure to clean up code and improve computational performance
• Define integration path for OpenMC into PyARC
• Cleanup PyARC by removing un-used capabilities PyPROTEUSMOC, PyTHCalcs, PARTISN
• Develop logic for every application to return ERROR message upon crash and stop PyARC