More DBBC2 IF filter choices in drudg

[wehimwich]

• Add a new skedf.ctl option for drudg like:

    DBBC2_ALLOW_ IF_FILTERS_3&4
  

with values yes and no. If the option is not present, the value should default to no. If yes, it would enable drudg to include IF filters 3 and 4, for Nyquist zones 4 and 3 respectively, in its selection of filters based on the BBC frequencies.

• Add a new skedf.ctl option for drudg like:

    DBBC2_ALLOW_1024_IF_FILTER
  

with the filter number number (1-8, typically 6) as the value. If not present, the usual filters options would be used. This would allow stations, with an LO that requires BBC frequencies that are in both Nyquist zones 1 and 2, to automatically select a filter that spans both zones when the situation occurs. This "cure" may be worse than dropping a channel and using a better filter. It would be up to the station, with advice, to decide which is better.

[wehimwich]

We have confirmation that filters 3 and 4 are safe to use generally and filters 5 and 6 should not be used at all. As a result, we don't need any skedf.ctl options for this. Instead drudg can consider filters 3 and 4 as acceptable choices along with 1 and 2.