Force inline of all boxdim methods

This boosts performance in CPU benhcmarks. GCC was not inlining minImage!
Simplify GPUArray allocate

Always allocate aligned memory on the host. This is needed for SSE/AVX optimizations.
Fix compile error
Default to -march=native

Also notify if built with SSE and/or AVX
Enable "import hoomd_script as blah"

When importing the hoomd_script module (not from import *), not all
the modules were imported. Rearrange the way the modules are imported
so that __all__ is not necessary and all modules are directly imported
when importing hoomd_script.