reuse computed I(qx,qy) values when computing 2D resolution

[pkienzle]

sasmodels/resolution2D.py sets the calculated qx,qy points by drawing NR x NPHI points around each measured point qx,qy. This means that the 2D pattern is computed 12, 30, 60, or 200 times depending on the choice of accuracy.

Rather than blindly drawing this many points around each qx,qy, we should first check if there are enough points already being computed in the pattern, and only add points where more are needed. This has the potential for significant speedup of 2D calculations when the resolution is worse than the pixel spacing.