refactor: separated "estimate mu" as suggested

[kkawato]

@sou412 @carolyyzz

This is an interim progress report, so there is no need to do a code review. I would ask you to review the entire code again after the cleanup.

• I separated estimate_mu as you suggested and created a new R notebook 1_2estimate_mu.R, although it is still messy. Also, following your advice, I did not save the results directly to data_all. Instead, I saved the output to data_test for each fold and merged them at the end.

• I was able to modify the Trycatch() part to be understandable by using if...else....

• As for the data_all and data_split part, the following suggested code did not work for some reason. I'm still trying to find out the reason...

data_all <- tempdata %>% .
mutate(fold_id = sample(1:fold, size = n, replace = TRUE))

data_split <- data_all %>% group_by(fold_id)
group_by(fold_id) %>%.
nest() %>%
arrange(fold_id)