iecorrect: allow for multiple id vars

[luizaandrade]

iecorrect template

• [x] make idvar() option mandatory
• [x] list ID vars as column names
• [x] test that ID vars are uniquely identifying

iecorrect apply

• [x] check that the ID vars are present in the sheet
• check that the ID vars are correctly filled
  • [x] if you fill one of them, you need to fill all of them
  • [x] if you don't fill any of them, you need to fill valuecurrent
  • [x] if you fill them, they need to match the type of variable (string or numeric) in the data
• in the do-file line, loop through all the id variables and
  • [x] check whether they are strings
  • [x] if they are strings, add " before the values
  • [x] otherwise, do not add "