add spath_assert

[adammoody]

I'm still trying to test this, but here is the idea of the assert and how one uses it in spath_bcast.

[gonsie]

@adammoody Any progress on testing this? Do you want to include it for the SCR 3.0 release?

[adammoody]

@gonsie , thanks for checking. It got stuck. It will have to come after v3.0.