Add warning for no observations within model domain

[mbruckner-work]

Right now the only indicator that there are no observations within the model domain is that the paired class will be missing data. In some cases due to how the pairing has been written, there will be model data in the paired object but no observations. The majority of cases should just return a blank paired object.

This warning is primarily needed for analysis of regional models. As an example, a TROPOMI NO2 and WRF-Chem comparison where the TROPOMI dataset does not contain orbits over the WRF-Chem domain currently produces a paired object with no observations and no indication as to why it failed.