Error in FCC paracrystaline model

[butlerpd]

Somehow in the major check, validation and fix to the paracrystaline models, one equation for the FCC was copied incorrectly: line 8 of sasmodels/models/fcc_paracrystal.c has the lattice spacing as the nearest neighbor divided by square root of 2 instead of multiply. This is probably because the similar equation for BCC has a division and the sloppy author (yours truly according to git blame) just cut and pasted and then changed the value of the denominator?

At any rate the peak positions for FCC are incorrect and should be corrected ASAP.