Feature parity with Matlab's timetable

[ValentinKaisermayer]

See Matlab documentation.

• Clean Data with Missing, Duplicate, or Nonuniform Times
  • [ ] ismissing
  • [ ] rmmissing
  • [ ] sortrows
  • [ ] unique
  • [ ] retime
• Resample and Aggregate Data
  • [x] isregular
  • [ ] retime

[chiraganand]

Checking isregular(), it was implemented in #130.