Created by: paulromano

After #1171 is merged, we no longer have any Fortran code in OpenMC. However, there are still some things that need cleaning up on the C++ side. I wanted to open this issue to keep track of them:

• Fix off-by-one indexing. Much of the C API still expects indices based on 1-indexing instead of 0-indexing. As a result, there is a whole bunch of -1 and +1s scattered through the code. We should consistently use 0-indexing now that there is no Fortran.
• Replace xyz and uvw with the Position and Direction classes (mostly as part of the Particle class)
• The Particle::initialize method should probably be part of a constructor?
• See if we can get rid of MAXCOORD and MAX_SECONDARY?
• Use the GSL library for spans (to replace pointer/length pairs in interfaces) and asserts
• Use trailing underscore for class data members consistently (especially Particle)
• Use enum class where it makes sense
• Use unique_ptr instead of raw pointers in global vectors

If any of you guys have other things in mind that I'm missing, feel free to add to the list.