adding portable code

Hi Melanie,

Trying out a pull request to include this portable code to manage paths on different machines. Hope this is relatively painless. You should only have to accept this pull request to transfer updated files from the master OHI-Science/ohiprep to your own working repository Melsteroni/ohiprep.

I just committed a little common.R file as part of this [ohiprep commit](https://github.com/OHI-Science/ohiprep/commit/7ec50f0561418fd5393d09fd6a8cd0d74920a6b9#diff-207250adc66b4fa693f7f5f61f7da5a8R6).

As an example, notice lines 6-24 Global/NCEAS-Regions_v2014/ingest_checks.R, especially  `source(common.R)` and use of `file.path(dir_neptune, ...)` .

Eventually, we'll probably want to make this a dedicated R package so we can have well documented functions. When we do the folder paths for the data products and code for manipulating paths will also change.

BB

PS Sending this to Julie too.