Implement third target: Cloud Droplet Number Concentration (cdnc)

[vasanchez16]

Another 2D model variable will be implemented for the training of more models. CDNC is a variable used to quantify the presence of clouds and can be useful for understanding the uncertainty of the model in capturing cloud physics and lifetimes (somewhat similar to CLWP).

The assimilation of this data should be straight forward and follow the steps of clwp (although with less calculation).

• Implement code to upsert the file urls in SppeSimulationEnsembleOutputFile
• Check grid to see if it matches coverage and resolution of other targets
• implement python code to calculate cdnc from weighted cdnc and weights
• add header to SppeSimulationOutputParameters

[vasanchez16]

All code is implemented. Testing and bug fixing is the next step