Include start date when saving diagnostics as NCDataset

[ph-kev]

When the diagnostics are saved as NCDatasets after a simulation, the start date is not available in the attributes.

The start dates should be saved in the attributes in the NCDataset so that one doesn't need to go to the simulation that generate the data and find the start date of the simulation.

[Sbozzolo]

We can fix this by adding a new argument to NetCDFWriter in ClimaDiagnostics. This argument will be something like more_attributes, a dictionary that maps keys to attributes, and we write those attributes as global attributes the first time we write the a NetCDF file.

[AlexisRenchon]

Done in #893 and ClimaDiagnostics CliMA/ClimaDiagnostics.jl#94 + CliMA/ClimaDiagnostics.jl#96