fixed a bug in the GPU improper calculation where "acos()" was called instead of  "acosf()"
fixed a bug in the GPU improper calculation where "acos()" was called instead of 
"acosf()"



git-svn-id: 9be72bd8b79bf6944680671de68cae08993c9086@1984 fa922fa7-2fde-0310-acd8-f43f465a7996

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue