read/write for `xmap_df` (using positional sorting)

[cynthiahqy]

• retain direction of crossmap and other metadata
• use positional sorting for from,to,weights, other cols

[cynthiahqy]

Names: #54

For WRITING

• xmap_to_csv(xmap, path)
• write_xmap_to_csv

For READING

• xmap_from_csv(path, from = NULL, to = NULL, weights = NULL, .keep = TRUE)
• read_xmap_from_csv

[cynthiahqy]

csv_to_xmap_df(file, col_from = NULL, col_to = NULL, col_weights = NULL, .keep_all = TRUE, ...) would just be a light wrapper around:

df <- readr::read_csv(file, ...)
col_names <- names(df)

from <- col_from %||% col_names[[1]]
to <- col_to %||% col_names[[2]]
weights <- col_weights %||% col_names[[3]]

xmap <- as_xmap_df(df, from, to, weights, .drop = !.keep)