Changes

gr%zm should be used here instead of gr%zt, since these are
momentum-level variables being calculated. Results are BIT_CHANGING.

Thermodynamic-level (zt) values should be used with zt-variable
calculations. This commit changes results (but not visibly)
for the l_avg_Lscale code, but since this code is not the default
in CLUBB, the code remains bit-for-bit since the last commit.