Technical debt: minor code cleanup in sw_fitpowder.m

[mducle]

This is just to record some TODOs for sw_fitpowder.m

• Refactor the planar polynomial background strategy to have separate polynomial coefficients vectors for the E- and Q-dependence rather than the current settings with polynomial coefficients for both E- and Q-dependence are combined in a single vector (with an offset index denoting where the E-dependent coefficients end and the Q-dependent coefficients begin), as suggested in this comment to PR#210.