syncing fork
JacobiWeightSpace sum

Generalized for both parameters via Sister Celine’s technique.
getindex override for Derivative
JacobiWeightSpace fix for length 1

found bug while running tests
clean up notation in addentries!

for Ultraspherical derivatives.
Merge pull request #36 from MikaelSlevinsky/master

JacobiWeightSpace sum and getindex override for Derivative