Follow-up from "Add "OrthogonalRecursiveBisection""

The following discussion from !95 (merged) should be addressed:

• @frey_m started a discussion:

  Why don't you use the members?

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In Index.h the types are as follows: int first_m; size_t length_m; Therefore the casting in Index.hpp for the 3 split functions is required to compile.. need to change member variables types in order to remove this casting.