lua/helfand_moment: fix construction
examples/shear_viscosity: remove unused force parameter
lua/stress_tensor_autocorrelation: remove breaking code
dynamics: add method connect() to TCF registration

The computation of the stress tensor autocorrelation requires
particle:aux_enable() to be called in sampling steps. The sampling
intervals, however, are unknown upon construction and not regular if
used in conjunction with blocking_scheme.

Let blocking_scheme:correlation() call tcf:connect() if such a method is
provided. It passes the sampling interval to the tcf module which
connects particle:aux_enable() to sampler.on_prepend_force. The
connection is managed (stored and disconnected) by blocking_scheme.

If sampler accepts arbitrary grids of sampling steps in the future, this
approach is easily extended: blocking_scheme constructs such a grid
and passes it to tcf:connect().
program_options: fix output of floating-point values

Default values for floating-point options were printed with an excessive
number of digits due to improper rounding, e.g., 0.1 was printed as
"0.10000000000000001".

Provide template specialisations to fix a short-coming of
boost::program_options relying on boost::lexical_cast.

http://stackoverflow.com/questions/5016464/boostlexical-cast-conversion-double-to-string-c
examples: use type float32 for cutoff option

The GPU implementation takes the cutoff as float argument. Thereby, the
default value of the "WCA cutoff", pow(2, 1./6), is reported with the
correct rounding and a reasonable number of digits.