sigcoh/sigcoh_util/formf.h beryllium form factor perhaps wrong

[ameliajo]

In form.h, we have

return 1 + cos( 2 * M_PI * ( 2 * l1 + 4 * l2 + 3 * l3 ) / 6 );
for a function that's supposed to return |F|^2/4.

In the documentation by General Atomics's HEXSCAT code (source below), on page 64 of the pdf (pg. 3 of the Appendix), it states that this should instead be

return 0.5 + 0.5 * cos( 2 * M_PI * ( 2*l1 - 2*l2 + 3*l3 )/6 );

Notice the difference in not only the 1 --> 0.5, but also the l2 term inside the cosine.

https://www.osti.gov/servlets/purl/4589776