fixed a bug with reading hermes climate data

- when during reading hermes climate data the input suddenly switched
from only sunhours to only global radiation the incomplete sunhours data
where read and would be out of range at some point
- now only complete sunhours data are added to the DataAccessor object
- additionally for documentation purposes an additional check for
sunhour availability has been added in Monica::cropStep to indicate to
the reader that it might be possible that no sunhours are available,
because due to the fact that for "completely" missing data elements will
always return 0.0 as a value upon getting a daily value instead of
throwing an exception or crashing or the like