For functions that allow users to select parameters (e.g., `est_change()`), allow users to select by operators

[sfcheung]

To-Do

• [x] Write tests for this feature.
• [x] Revise the functions to let users select parameters by supplying a vector of operators, e.g,. c("=~").
  • [x] est_change()
  • [x] est_change_raw()
  • [X] influence_stat(): No need to change it. It pass parameters directly to est_change(). Let est_change() handle the change.
• [x] List functions that allow users to select parameters.