ENH: Handling multiple calls to cosmotools_init

This commit adds more graceful handling of multiple calls
to cosmotools_init. Instead of leaking silently, calling
cosmotools_initialize multiple times will now throw a
runtime_error.

From: George Zagaris <george.zagaris@kitware.com>

git-svn-id: https://svn.alcf.anl.gov/repos/DarkUniverse/hacc/trunk/cosmotools@1995 952bdfbe-fa2e-45c0-a9ae-ac90d4a62daf
Merge branch 'svn/trunk'