Adds dim checks and tests

- Adds NcDimUnlimited exception
A new exception to handle the case where a variable is to be written
along an unlimited dimension but the bounds of the array are not
specified.

- Renames NcFile.close -> NcFile.Close

- Implements NcGroup.AddVar

- Adds Dimension length checking to NcVar.PutVar(number[])

- Adds test for unlimited dimension writing

- Adds test for extended use case
  - New file with 3 dimensions
  - One dimension of unlimited size