Refactor creation of required dataframes

[dave-mills]

Currently, many of the graphics functions require certain dataframes to already exist, particularly indicator_table, data_dictionary and code_lists.

One approach to this is to make getting these data their own functions, which can then be called in any context (at the start of a processing script; inside a function, where-ever). For example:

get_indicator_table <- function(con) {

    if(exists('indicator_table') && is.data.frame(get('indicator_table'))) {
        return indicator_table
     }

    return tbl(con, 'indicators') %>%
        collect()
}

Now, anywhere we need the indicator_table data, we can run indicator_table <- get_indicator_table(con). If the dataframe exists in the current environment / context, it will be used. Otherwise, we go out to the database to get it.