Updates to Sigma

Sigma has a domain and range space.
Addentries! is updated for all ultraspherical spaces since they’re
orthogonal with the weight (1-x^2)^{\lambda-1/2}.
Included Sigma tests.
Nonnegative ultraspherical spaces
typo in tests
LowRankFun and Fun algebra
Correct parameterization
Merge pull request #75 from ApproxFun/feature-update-Sigma

Feature update sigma