Add constant for invalid surface tokens.

[pshriwise]

Description

As I was going through some of the plotting code I noticed that through out the code we use a value of 0 for an invalid surface token. Here, token meaning a signed, off-by-one index as opposed to a signed surface ID.

The "magic number" is a little confusing when we're constantly converting between tokens and indices into the model::surfaces vector. Further, we initialize BoundaryInfo.surface_index to 0 because this member is really a token rather than an index.

In an effort to distinguish surface token values I've added a SURFACE_NONE constexpr that is 0 for these cases.

I've also updated BoundaryInfo::surface_index attribute to surface in an effort to be consistent with the GeometryState::surface_ attribute and added BoundaryInfo|GeometryState::surface_index methods that convert from the token to the index for that token. This doesn't cover all of the cases where we do this conversion, but I think it generally increases clarity were surface indicates a token and surface_index an index.s

Update

It's worth noting that there's a fix for the surface index passed into the DAGMC next_cell call for projection plots. The off-by-one used to be handled in next_cell but the argument passed in was already adjustment, so this was happening twice.

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) N/A
• 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) N/A

[paulromano]

Love the change here! One question though about the use of next_cell for DAGMC geometries: