Extended precision polynomial construction

[kburns]

This PR modifies dedalus_sphere to implement double-double extended precision with the xprec package, related to #166. This seems to work well but doesn't currently fix range/underflow/overflow issues in the construction of the SWSH, since xprec currently only supports extended mantissas but not extended exponents. However there may be some other options using other packages, but none seem to interface very well with numpy or scipy.sparse.