add simualte length age conversion code

tests/testthat/fixtures/simulate-length-test-data.r

@@ -0,0 +1,48 @@
+working_dir <- getwd()
+
+maindir <- tempdir()
+
+AtoL <- function(a_f,Linf_f,K_f,a_0){
+  L <- Linf_f*(1-exp(-K_f*(a_f-a_0)))
+}
+
+amax <- 12
+Linf <- 80
+K <- 0.18
+a0 <- -1.36
+amin <- 1
+cv <- 0.005
+ages <- 1:amax
+len_bins <- 22:75
+
+mean_length_age <- AtoL(1:amax,Linf,K,a0)
+
+om_input_list <- list("amax" = amax, "Linf" = Linf, "K" = K, "a0" = a0, "amin" = amin, "cv" = cv, "ages" = ages, "len_bins" = len_bins) 
+
+length_age_conversion <- matrix(NA,nrow=length(ages),ncol=length(len_bins))
+for(i in seq_along(ages)){
+  #Calculate mean length at age to spread lengths around
+  mean_length <- AtoL(ages[i],Linf,K,a0)
+
+  #Calculate the cumulative proportion shorter than each composition length
+  temp_len_probs<-pnorm(q=len_bins,mean=mean_length,sd=mean_length*cv)
+  #Reset the first length proportion to zero so the first bin includes all
+  #density smaller than that bin
+  temp_len_probs[1]<-0
+  #subtract the offset length probabilities to calculate the proportion in each
+  #bin. For each length bin the proportion is how many fish are larger than this
+  #length but shorter than the next bin length.
+  temp_len_probs <- c(temp_len_probs[-1],1)-temp_len_probs
+  length_age_conversion[i,] <- temp_len_probs
+}
+colnames(length_age_conversion) <- len_bins
+rownames(length_age_conversion) <- ages
+
+on.exit(unlink(maindir, recursive = TRUE), add = TRUE)
+
+setwd(working_dir)
+on.exit(setwd(working_dir), add = TRUE)
+
+save(om_input_list, length_age_conversion,
+  file = test_path("fixtures", "length_test_data.RData")
+)