FIX - Coverage constructor cannot create multibands in netCDF

[Bang Pham Huu]

For example this query:

for $c in (test_mr)
return
  encode(
    coverage mycov
    over $e E(0:0), $n N(0:0)
    values {
      band1: (unsigned char)($e);
      band2: (unsigned char)($n);
      band3: (unsigned char)($e+$n);
      band4: (unsigned char)($e-$n)
    }
  , "netcdf")

generated this rasql query with only 1 band

SELECT encode(MARRAY e in [0:0,0:0] VALUES { (char) ( e[0] ) , (char) ( e[1] ) , (char) ( e[0] + e[1] ) , (char) ( e[0] - e[1] ) }, "netcdf" , "{\"dimensions\":[\"E\",\"N\"],\"variables\":[{\"type\":\"double\",\"data\":[0.5],\"name\":\"E\",\"metadata\":{}},{\"type\":\"double\",\"data\":[0.5],\"name\":\"N\",\"metadata\":{}},{\"type\":\"double\",\"name\":\"band1\",\"metadata\":{\"units\":\"10^0\"}}],\"nodata\":[]}") FROM test_mr AS c