Add support for saving time varying nodal data

[krober10nd]

• Nodal data is saved using the time obj that the Variable was built with in a CF-compliant form.
• What reference date should be used? I used 1970-01-01 00:00 UTC but that won't work for all applications.
• Still need to check all Variable objects have the same number of time steps or throw an error.
• Add support for face and edge based data.