[ENH] single join fully implemented in numba (#1304)

* single join fully implemented in numba

* get single equi join in numba; check for monotonicity before sorting

* improve logic for monotonicity in numba equi join

* add tests

* simplify boolean capture logic

* restrict column check to only non-equi joins, or if use_numba=True for equi joins

* add docstrings to tests

* update dtype check

* simplify dtype check further

* Update conditional_join.ipynb

* add validate keyword for equi joins

* fix docstrings

* further fix docstrings

* remove validate keyword

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Co-authored-by: Eric Ma <[email protected]>
Co-authored-by: samuel.oranyeli <[email protected]>