Look at B+tree

[tzaeschke]

There may be some issues with the B+tree:

• [x] Is rebalance() called correctly?
• [ ] Should we use delegation i.o. inheritance? Avoid vptr and virtual destructor? Alignment/packaging?
• [x] Test copy/move constructors
• [ ] Is using std::pair the wisest choice, i.e. it's sometimes not intuitive whether it calls copy or move on values..., e.g. https://stackoverflow.com/questions/58283694/why-is-pair-of-const-trivially-copyable-but-pair-is-not

[tzaeschke]

rebalance() was fixed in #114.

[tzaeschke]

copy/move was fixed in https://github.com/tzaeschke/phtree-cpp/pull/119.