Add nbnxn tree force reduction

Alternative implementation of the nbnx force reduction. The previous
method is implemented as a loop over all thread buffers. Its advantage is
that it doesn't require any synchronizations during reduction and only
requires a single write per element. The new method utilizes a binary tree
for the reduction. Its advantage is that the data locality of the force
buffers is used during reduction.

The current implementation isn't faster on the CPU (for E526070 with icc).
On the MIC it is faster (36% reduction of buffer-f time for 32 threads).

Part of #1420

Change-Id: I2d84345e9a19d9030a837d324671f2ab0ba9b91b