the sphinxcontrib.image extension was broken with sphinx 16.x.
however, sphinx 15.x breaks with newer version of the multiprocessor module.
so we suspend the thumbnail processing and lift the lock to sphinx 15.x

also, the number of parallel sphinx tasks is can be overridden with SPHINXEXTRA="-j #'.
default is to try use all local CPU cores.

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@15668 f3b2605a-c512-4ea7-a41b-209d697bcdaa

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@15590 f3b2605a-c512-4ea7-a41b-209d697bcdaa

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@15561 f3b2605a-c512-4ea7-a41b-209d697bcdaa

(cherry picked from commit 79e867c213ee022685e5bb8a5089a112099e4f06)

(cherry picked from commit 4629a464f701f919633fd220860b6610d097c291)

(cherry picked from commit 453521c8e682c6563226dfcc57c689052dacd21f)

(cherry picked from commit 7cb39811d47b1f364286597acea21c3afd010733)

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@15103 f3b2605a-c512-4ea7-a41b-209d697bcdaa

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14950 f3b2605a-c512-4ea7-a41b-209d697bcdaa