iecorrect: add column to specify number of observation being dropped in tab "drop"

[luizaandrade]

The command should then check that the number of observations being dropped is as expected, and throw an error if not, returning the unmodified data set

[luizaandrade]

• Include a new column in the template (n_obs)
• Throw an error if this column is not filled
• Before dropping any variable, test that the number of observations that will be dropped is correct
  • If it is not, throw an error message and don't make any changes at all (don't run iecorrect)
  • If it is, just print a message like "X many observations will be dropped"