thermodynamics: return actual stress_tensor

Instead of the stress tensor divided by the number of particles, return the
actual extensive quantity as it is defined in e.g. the book of Hansen
and McDonald (Ch. 8.4).

This will simplify the calculation of the shear viscosity.
sampler: add start parameter for on_sample()

The 'start' parameter makes it possible to delay the call of the slot provided.
A usecase for this functionality is the acquirement of e.g.
thermodynamic variables skipping a certain time at the beginning.
numeric/accumulator: Support vector types, add sum()

Previously, only types that implement T operator*(T, T) were supported. By
replacing the multiplication with element_prod() this allows the use of
fixed_vector.
For support of types that implement operator*(), provide a
element_prod() variant that simply calls this multiplication.

Also add a function sum() for the accumulator that returns the sum =
mean × count.

Update the unit test to reflect these changes.

In order to support fixed_vector<dsfloat>, add template specialisation
for boost::is_floating_point<dsfloat>.
Add new observables/accumulator module

This module is a mere wrapper for numeric/accumulator with additional
lua exports.
lua/thermodynamics: provide dimension and group attribute

Make these information available in the thermodynamics lua module as
they can be useful for other modules.
observables: provide helfand_moment module

This module calculates the helfand moment from the integral over the
stress tensor in time.
observables: add stress tensor autocorrelation function

This module works analogue to the velocity autocorrelation function.
add example for shear viscosity using the new modules
minor revisions of observables/…/accumulator module

* revise interface of writer method to be more generic, anticipating
  the upcoming changes in phase space writer

* make uint64_t known to the H5MD writers to support the output of
  "count"

* rename error() to error_of_mean() as in numeric::accumulator

* fix misplaced 'group' argument in example script
observables: revise helfand_moment docs, change normalisation
observables/dynamics/stress_tensor: change normalisation

Slightly revise documentation.