install pgdb inside of custom directory

[james-tanner]

When pgdb install is called with a custom directory, the current behaviour will use the provided directory (and delete any content inside that directory).

This seems like quite dangerous behaviour to me, as I suspect the intuition of most users would be that pgdb would be installed inside the provided directory -- i.e. if one provides /Users/johndoe/Research/, pgdb should install itself at /Users/johndoe/Research/pgdb.