Allow MeshSource to take a 1D array of sources

[paulromano]

Description

Right now, the MeshSource class requires that when you provide sources for each element of a structured mesh, you pass it a multidimensional array that has the same shape as the mesh. So, if you have a RegularMesh with mesh.dimension = (10, 2, 5), then the array of sources also has to be of shape (10, 2, 5). This PR allows you to pass a flat array of sources in the order of the mesh element indices. This makes it a more mesh agnostic since the same interface can be used for both structured and unstructured meshes (passing a flat array).

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[eepeterson]

Thanks @paulromano!