scipy.special.orthogonal.p_roots with cache
the mu parameter can also be a float
Revert "the mu parameter can also be a float"

This reverts commit 48674c2835861a2ce6a663f94fbc95f5cccdd663.
Revert "scipy.special.orthogonal.p_roots with cache"

This reverts commit 431133df5340c923dc4887d570faf01370869a8a.
_cached_p_roots for speeding up quadrature
Merge pull request #3919 from Horta/master

ENH: integrate: quadrature speed-up by caching orthogonal.p_roots