New analysis tool to compute the free volume and total volume.

Works with selections, such that the freevolume in a box containing
a subset of all atoms can be computed.
Added excessive comments so that others can use this as a reference
for implementing analysis tools as well.
Added a test system as well, but this is somewhat tricky due to
real precision and the Monte Carlo algorithm.
literature reference.
valgrind.

Change-Id: I2acb48b829c9495e399c60e9dc927662e2958d69