Support source files for fixed source simulations

[paulromano]

Right now, the only way a source file can be used is to fill the source bank used for k-eigenvalue simulations. It's not currently possible to use a source file in conjunction with a fixed source simulation. It seems like a straightforward way to allow this would be to have the SourceDistribution class itself keep a reference to the source file (rather than having it as a global variable as we currently do) and using that to get a source site from the file when it is sampled.