BUG: stub out spurious methods of Akima1DIntgerpolator

from_spline and from_bernstein_basis are inherited
from PPoly, and neither produces an Akima interpolator.
DOC: add See Also sections to Pchip and Akima interpolators
Merge pull request #3744 from ev-br/akima_stub

stub out inherited methods of Akima1DInterpolator