Diagnostic variables and thread safety

[fmaussion]

There are a few problems occurring when working with open_mfdataset and salem's custom netcdf4 objects. Here's a couple of the nice messages one encounters:

python3.4: posixio.c:342: px_rel: Assertion `pxp->bf_offset <= offset && offset < pxp->bf_offset + (off_t) pxp->bf_extent' failed.
python3.4: posixio.c:265: px_pgin: Assertion `*posp == ((off_t)(-1)) || *posp == lseek(nciop->fd, 0, 1)' failed.

Or, on Travis:

/home/travis/build.sh: line 57:  1696 Segmentation fault      (core dumped) 

I'm not sure how to deal with this yet...

See also: #36