First commit adding fixed source calculations

[kevinm387]

Hello @rvignolo, I'm submitting this PR to try to add the capability of running fixed source calculations to NeutronTransport.jl. The functions and methods added are similar to OpenMOC's computeSource() and setFixedSourceByMaterial() functions. I believe I may have some bugs present still, but I want to submit the PR to get your input. I included an example problem based on the pincell demo problem. Thank you!