Implement "Add Appointment" feature

[yadobler]

Feature: Add Appointment

Purpose: Allows doctors to schedule an appointment with a patient.

Command format: addAppt ApptName i/NRIC @d/DATE @t/TIME

Example commands: addAppt Physio i/S1234567A @d/2024-09-20 @t/1000-1100

Acceptable values:

• ApptName:
  • Must be Alphanumeric.
• NRIC:
  • Must be 9 characters.
  • first and last characters being alphabets.
  • Follows the 9-character Singapore NRIC format for uniqueness.
  • Case-insensitive (e.g. ‘TXXXXXXXB’ is the same as ‘tXXXXXXXb’) for consistency.
• Date:
  • Must be in the YYYY-MM-DD format (e.g., "2024-09-20").
  • Standard YYYY-MM-DD format for consistency and scheduling ease.
• Time:
  • Must be in the HHMM format (24-hour clock) (e.g., "1000")
  • must be in an interval (e.g. 1000-1100).
  • Appointment time intervals cannot overlap with other existing appointment time intervals.
  • Standard HHMM format for consistency and scheduling ease.
  • Time format should be (start time - end time) where start time < end time. The start time should not equal the end time.

Error message if invalid:

• "Invalid date format. Use YYYY-MM-DD."
• "Invalid time format. Use HH:MM."
• "Patient does not exist.”
• “There is an existing appointment for this date and time”

Outputs:

• Success: "Appointment for John Doe scheduled on 2024-09-20 at 10:00."
• Failure: Error message related to the incorrect input format.
• Duplicate handling: The system will check if an appointment already exists for the given date and time interval, prompting a message like "Appointment already exists for this date and time."