Implement Singleton Pattern for XSLibrary

[bkochuna]

Right now the populateXSec method on the Material class takes a pointer argument for the XSLibrary class. Having this class as an argument complicates creating a C interface for the class.

Further, in 99% of our use cases, we will most likely have only 1 cross section library for all materials. Therefore, it should be beneficial to implement the Singleton pattern for the XSLibrary class. In doing so, we can then simplify all interfaces that take XSLibrary class as an argument.

To complete this issue, the following tasks should be completed.

• Implement singleton pattern for XSLibrary
• Update unit testing for XSLibrary
• Update clients of XSLibary to access singleton and eliminate dummy arguments
• Update appropriate unit tests

Definition of Done

The aforementioned tasks have been completed. The code has been reviewed. The checks have passed. The branch has been merged.

[KyleVaughn]

This sounds good to me. Since the XS library is somewhat hidden from users, it may be worthwhile just to implement this as a global variable and save yourself some hassle. My experience lately has been that enforcing a true singleton is more work for developers than it's worth if users cannot realistically instantiate a second object, especially if the object is used in many functions.

One of the few cases where a global makes sense in my opinion. Happy with a singleton instead if you prefer.