Converts row & column based area references to Excel area references

`idx2aref(x)`

x

Numeric (integer) matrix or vector of indices. If a matrix is provided it should have four columns with the first two columns representing the top left corner (row and column indices) and the third & fourth column representing the bottom right corner. If a vector is provided it will be converted to a matrix by filling the vector into a 4-column matrix by row.

Returns a `character`

vector of corresponding Excel area references.

# NOT RUN { idx2aref(c(1, 1, 5, 4)) # }