Update `fill` command to have sane default execution

[Carsons-Eels]

What was wrong?

When fill is called without any arguments it fails because there is no target directory or sane flags. This should not be the case.

How can it be fixed?

At the very least, calling fill without any arguments should print a usage message and exit. In the best case (IMO) it should run some default configuration that strikes a balance between commonality of use and execution time.

[Khushi-Chaudhary04]

Hi! I'd like to take this on. I'll focus on implementing the clean exit/usage message when no arguments are provided, as you suggested. Should have a PR ready soon! Thanks!