Interview PED fix

[foochifa]

Can be Merged

Changes:

• Improve Sample Data
  • mutable list given to interview list
  • Interviews in status.Interview, and removed interviews from others.
• Interview.java is abstract
  • OnlineInterview and OfflineInterview inherits from it
• InterviewAddCommand and InterviewEditCommand now ensures interviewDate > applicationDate
• Improve error messages for InterviewCommands

Todo:

• property change for interview list

resolves #196, resolves #193, resolves #174, resolves #162, resolves #165, resolves #160

[ongch00]

Looks good, Thanks!