Reorganizing analysis of correlation functions.

Reorganizing analysis of correlation functions.

Moved routines computing correlation functions and
fitting to those in a special subdirectory not
dependent on gmxana. The autocorrelation now functions
as it should (previously there were issues with long
time correlations).

Replaced Levenberg-Marquardt algorithm by another implementation
from http://apps.jcns.fz-juelich.de/doku/sc/lmfit,
under a FreeBSD license. This code is in src/external/lmfit.

Added Doxygen comments to all headers.

Fixed wordcount algorithm in xvgr.c

The routines many_auto_correl and many_cross_correl
are optimized using OpenMP.

Change-Id: Ifbe003437db36e35d606cd942bb38deeef86ea64