Added robot model interface class

Addresses issues #8 and #9.
This is something that probably should've been discussed much earlier. but the type std::size_t used for the "id_" members will only allows us to assign positive values starting at zero.  This could be somewhat limiting if we had to check for ID validity by comparing its value to a range of valid id values.  If "ints" were used instead, we could default this ID value to -1 or less that zero which indicates that the point has not yet been indexed or initialized properly.