Fix crash during cube-merging when one or more of an AuxFactory's
dependencies is set to None.
Allow HybridPrssureFactory to represent formulae of the form
  p = a * p0 + b * ps
as well as
  p = ap + b * ps
Add pyke rules and related code to automatically generate a
HybridPressureFactory for hybrid sigma pressure vertical coordinates
in NetCDF files (as already done for hybrid height coordinates).