Multiple changes from Amber GIT:
-Keep track of individual native contact stats in nativecontacts, including optionally time series. Combine some common functionality into macros. Contacts are now stored using map instead of set for efficiency.
-Split contacts map into native and non-native contacts, which is much easier to read.
-Add ifdef for systems that do not support _SC_XXX variables for sysconf().
-Add code to determine the available (free) memory on a Mac.
-Add normframe and normdensity options to grid for normalizing by # frames or # frames and specified molecule density respectively. Add function to GridBin for calculating voxel volume.
-Frame::Center -> Frame::CenterAtoms to avoid confusion with other routines.
-Fix autoimage centering to box center for general triclinic cells.
-Use CenterOnOrigin instead of CenterAtoms, latter is overkill.
-New boxcenter function in vector.
-Update test for fixed triclinic autoimaging.
-Move centering functionality back to Action_Center since nothing else needed it.Get rid of obsolete enumerated type for centering in Frame and corresponding unneeded code in Action_Autoimage.