BLOSUM62 shouldn't be greyed out if consensus/quality/conservation calcs are disabled

If all calculation annotation rows are disabled in preferences and a protein alignment is loaded in to Jalview, the BLOSUM62 colourscheme is disabled because the background conservation thread hasn't been started.

This behaviour is inconsistent - since PID and Clustal styles are still available (and work).

The fix here would be to automatically trigger the calculation when BLOSUM shading is selected, just like with the other colourschemes, rather than simply disable it.