Implement lot_t::insert()
Add date to car_t ctors
Implement add_car
Explain obscure code in lot_t::insert()
Add begin_by methods to lot_t for iterator access
Make lot_t::begin_by_* iterators const_iterators
Mark lot_t::begin_by_*() / end_by_*() const
Prefix lot_t::begin_by_*() / end_by_*() with c

to denote their const-ness
Implement print_lot
Fix grammar in comment
Add calls to print_lot() at exit_car and print_lot
Fix formatting for car_t << operator
Remove blocking call to cin.ignore()