Validation

A new function that will check whether the input is correct. So far, the
function checks the fields with the following requirements:

-Course name: must not be empty
-Course number: must be a four-digit number
-Start time: must be in the millitary 24-hour format from midnight to
2359 (11:59)
-End time: same time format requirement applies.

TO_IMPLEMENT: check if the last two digits of the time ar not greater
than 59.

-Dima