ppc_loo_pit_overlay(): NA values generated near ends after convolution in boundary correction

[ecoronado92]

Currently boundary correction generates NAs for values near ends of vector after convolution . Important to note that # of NAs "varies" so na.omit() won't work and will cause size inconsistencies. Current fix, use last non-null value at head and tail of vector as padding.

See line 627 in R/ppc-loo.R