comment on test_normal failing if r8=r4

https://github.com/NCAR/DART/pull/788/files/be7f568f4faad4cafefb12a21160609d3f84cc54..b9e67b4e214162de8c5b47db418f5112cbe30833#r1904307694

assimilation_code/modules/assimilation/normal_distribution_mod.f90

@@ -160,7 +160,7 @@ function inv_weighted_normal_cdf(alpha, mean, sd, q) result(x)
 ! VARIABLES THROUGHOUT NEED TO SWITCH TO DIGITS_12
 ! The comment above is not consistent with the performance of these routines 
 ! as validated by test_normal. There is no evidence that this is still
-! required.
+! required. HK @todo test_normal fails if r8=r4, so beware.
 
 ! Can search in a standard normal, then multiply by sd at end and add mean
 ! Divide q by alpha to get the right place for weighted normal