removed condition that seemed to create very large support
in sum hills calculation and resulting into a very large computational
time. Credits to Carolyn Phillips from the mailing list.

(backported from master, see commit 92717cb6)

Also fixed bug in KernelFunctions

In doing so I fixed a number of bugs in the KernelFunctions
class.  The derivatives calculated by the evaluate function
in this class were wrong.  Tests still need to be done to check
whether or not the derivatives are correct when non diagonal
metrics are used.

(cherry picked from commit 5102dc33)

I set it to 2 instead of 2.0, so that it will not be necessary
to update it

and sum_hills: added --outfile and --outhisto

If you want to but a plumed_assert in a loop you should now use
plumed_dbg_assert / plumed_dbg_massert.  This will not be included
in the code if you compile with -DNDEBUG

I used cppcheck --enable=all.

When using IFile or OFile only, it is sufficient to include
one of the two header files.

Also, consistently:
PlumedIFile -> IFile
PlumedOFile -> OFile
PlumedFileBase -> FileBase

The guard "ifndef/define" has been updated so as to also include the
module name. This is important as it will allow files with the same name
to be used in different directories. Modification was done automatically
using header.sh - just run it routinely to keep names consistent