REF: move most of Block.interpolate to an array method
Not _quite_ at the point where it can be called directly from ArrayManager, but close.
@jorisvandenbossche have you gotten a chance to look at the Categorical-ArrayManager failure on master? any issues with AM tests @jbrockmendel ? > any issues with AM tests

nothing showing up in the CI.  AM doesnt (yet) call this directly > Not quite at the point where it can be called directly from ArrayManager, but close.

What's needed additionally to have it usable for ArrayManager? > What's needed additionally to have it usable for ArrayManager?

the short-circuiting on L1113 and L1123 need some special attention (particularly the one on 1123).  Then maybe_downcast at the end needs porting (ive got a branch but am not quite happy with it yet) can you merge master and ping on green ping