Define an S3 class and related function for input

[elong0527]

replacing the exposed tibble() and data.frame() calls for constructing non-trivial types of inputs with formally-defined classes and generators using S3 or else.

Without it, we are just assuming: 1) the developer always know what an input object has; 2) users always know what they should put into them; 3) users can construct them correctly.

Reference: https://adv-r.hadley.nz/s3.html

Example: ggplot2::aes https://github.com/tidyverse/ggplot2/blob/main/R/aes.r