Random Ray Solver Bug When Multiple Point Sources in Same Source Region

[jtramm]

Bug Description

Currently if there are multiple point sources that end up residing within a single subdivided source region, the random ray solver will only use the one with the higher index in the model::external_sources array.

We should change it so that either an error is given, or the sources are supported correctly. My thinking is that PR #3114 will remove the need for this sort of source treatment, so not worth upgrading the data structures to handle this situation.