Validator should indicate when time series do not overlap

[cchwala]

When get_time_series gets called in a GridValidator and the CML time series does not overlap with the one of the validator object, a cryptic error is thrown.

It would be better to raise an exception here that clearly states that there is no overlap.

This could be done here

pycomlink/pycomlink/validation/validator.py

Line 62 in 5467529

t_mask = (self.xr_ds.time > t_start) & (self.xr_ds.time < t_stop)

by checking if t_mask contains only NaNs