makesourcedb appends skymodel by default

[henedler]

According to the makesourcedb doucmentation, the default behaviour when creating a sourcedb that already exists should be to overwrite this sourcedb.

However, it seems that by default, the skymodel is appended when running the same makesourcedb command multiple times.

[revoltek]

In general I think the makesourcedb step should be removed and DP3 should read txt skymodels and do the conversion internally