Added validation code to Qso2.Validate to ensure that a QSO has
at least the minimum fields: call, mode, band or freq, qso_date,
and time_on.
Added and modified tests in Qso2Tests and Qsos2Tests for this change.
Added ToAdifString method to AdifField class. Removed ToAdifString
method from a number of the derived classes and added override
keyword to method in remaining derived classes.
Added ToAdifString method to Qso2. Added and modified tests to match.