Minor fix to the creating of `outdir`

Previously, outdir was checked and if-not-existing created in a few different places in two different ways. This (a) makes them all use the same method (namely the utils function) and (b) places a check-and-creation call before every tupak output. This will ensure that changes elsewhere in the code won't cause bugs in the future. Also, if you choose not to save anything, it won't just make an empty directory.