`meta_split()` split metadata only by variable from population

[wangben718]

Currently, meta_split() is splitting metadata by unique value from population dataset.

metalite/R/meta_split.R

Line 41 in a319e1d

for (i in 1:length(pop_by)) {

However, the same column, i.e., race, which is included in both population and observation dataset, could have different value. For example, both "Asian" and "White" are in population but only "Asian" in observation data. This will cause an error when splitting metadata by this variable.