Alan gsynth fix

[sideye]

This PR should now have fixed all errors and warnings when running devtools::check:

• Fix gsynth to run with gsynth instead of gsynth_core for both single and multi outcome
• Append test case for gsynth to test for correct results.
• Fix logic in multisynth_formatted concerning n_factors and throw an error if not enough pre treatment periods exist when doing CV to select n_factors.