Add Cf compliant time to core_atmosphere

[dimomatt]

CF compliance in time axis requires a unit of ' since YYYY-MM-DD hh:mm:ss'. This commit creates the 'Time' variable in the core_atmosphere Registry.xml, initializes it to 0, and changes the 'units' attribute to 'seconds since '. The start timestamp is modified from the initial xtime format to remove the underscore and replace it with a space to match the CF compliant time string format.

We can then start to update the Time variable at the same time as the xtime variable by adding 'dt' to the Time at time level 1, and saving it to time level 2. This produces a CF-compliant time variable that tracks xtime at each step.

[mgduda]

After more testing, I think there are a few additions that we may like to include in this PR:

• [x] Add the CF-compliant Time variable to all default output streams in the atmosphere core
• [x] Add the standard_name attribute with a value of time to the Time variable
• [x] Ensure that the value of the Time variable is correct for each of the time slices in surface update files produced by the init_atmosphere core
• [x] Ensure that the value of the Time variable is correct for each of the time slices in lateral boundary files produced by the init_atmosphere core