Tidy a sparseMatrix object from the Matrix package into a three-column data frame, row, column, and value (with zeros missing). If there are row names or column names, use those, otherwise use indices
# S3 method for dgTMatrix
tidy(x, ...)# S3 method for dgCMatrix
tidy(x, ...)
# S3 method for sparseMatrix
tidy(x, ...)
A Matrix object
Extra arguments, not used