Write a generic index plot function

[sfcheung]

To-Dos

• [x] Write a test.
• [x] Draft the function.

Status

Working in the branch generic_index_plot.

Note

This function allows user to use any column in the output of the main function to produce an index plot.

• It should allow user to plot the absolute value (e.g., for case influence on parameter estimates) or raw value, which can be positive or negative.

• We can consider revising other specific functions to call this generic function internally.

[sfcheung]

Close #101