convert a matrix to a tidy data frame (from wide to long format as described in the tidyverse concept)
mat2df(x)
a data.frame in long format with the 'value' column stores the original values and 'row' and 'col' columns stored in row and column index as in x
x <- matrix(1:15, nrow = 3)
mat2df(x)
#> value row col
#> 1 1 1 1
#> 2 2 2 1
#> 3 3 3 1
#> 4 4 1 2
#> 5 5 2 2
#> 6 6 3 2
#> 7 7 1 3
#> 8 8 2 3
#> 9 9 3 3
#> 10 10 1 4
#> 11 11 2 4
#> 12 12 3 4
#> 13 13 1 5
#> 14 14 2 5
#> 15 15 3 5