Streamline scratch logic

[mscho527]

          oh, i meant something slightly different -- i thought it would be useful to do a logic where

1. if BE.scratch_dir was set, use this in the solver
2. otherwise, use the input keyword
   but i think we can address this in a later pr (reminder to create an issue upon merge) as this is okay as is

Originally posted by @mscho527 in #41 (comment)

[ShaunWeatherly]

It's currently setup in the reverse, ie if you set the input input keyword scratch_dir then that supersedes whatever is set in BE.scratch_dir.

[oimeitei]

why not just use BE.scratch_dir instead of introducing a new input keyword? you can set BE.scratch_dir in an input rather easily.