Working Define CF Model dialog and NXspectrum fix

• The NeXpy "Define CF Model" plugin dialog is now functional, with a parameter grid for the rare earth, symmetry, Stevens parameters, Hz/Hx, and temperature, plus Load/Plot/Save actions and "Overplot Spectrum"/"Plot as Line" plot options.
• from cfcal import CF now works — CF is exposed at the package top level.
• Fixed a TypeError raised by CF.NXspectrum() when no eps energy grid was given; the default grid was never being computed.