Better check of available meteodata

[jlandmann]

Now, in the operational module, we just check if the date of yesterday is available in the climate file.
It would be way better to check if all variables, i.e. temperature, precipitation and soon radiation are there => if one is not present, the retry decorator should catch this case.
If radiation is not there, one could start the radiation-independent mass balance calculations. However, this might be a bit advanced for the moment....