Make 'gmx sasa' calculation thread-safe

Remove use of all static/global variables from the surface area
algorithm in surfacearea.cpp.  This makes 'gmx sasa' computation mostly
thread-safe (only the Connolly surface output isn't).  This in turn
gives the first analysis tool that does a non-trivial amount of
computation per frame in a thread-safe manner.  Once we have TBB or some
other proper threading library in use, it should then be straightforward
to also implement threading support for the analysis framework, and
actually have a tool where the benefit could be observable.

Simplify the implementation a bit, as there were quite a few state
variables in the orignal code that were never used.

Change-Id: If9fa4829620afbe09c1e206903190ba03af8a082