observables/dynamics/blocking_scheme: fix doc

The separation parameter is given in sampling steps, while `every` is
given in integration steps.

Add logging info on the sampling interval.
observables.thermodynamics: adjust output to H5MD spec

The dimension, particle number, and box volume are always stored in a
thermodynamics group. Use of the H5MD module "thermodynamics" has to be
declared in the metadata.

Rename nparticle() to particle_number() to reflect the H5MD name of the
field.

Bind data slot volume() to Lua. Do not query mdsim.box as the volume of
a particle group may be smaller than that of the whole box.

This commit refs #198804.
box: drop output to "/observables", read from "/particles"

In H5MD, the box is no longer mandatory in "/observables", but
accompanies each subgroup in "/particles".

Write the box as fixed-in-time data.

Adjust comparison of box data in continuation test.

This commit refs #198804.
io.readers.h5md: clean-up Lua script, emit error message in check()

Emitting the error message here helps simplifying the user script. If
needed we may introduce an optional parameter to suppress the error
message.
thermodynamics: remove evaluation of hypervirial

The hypervirial can be used to determine the compressibility. The
results, however, suffer from poor statistics and a superior method is
provided by extrapolation of the static structure factor to zero
wavenumber.

The method has been marked in commit:deec62b as "to be deprecated" for
the next release (after 0.3). Dropping it now is actually not a good
habit, but the quantity was not used by anybody so far to my knowledge.
observables/sampler: reintroduce on_prepare() signal

After being removed in commit 9360386, it is needed for the new forces
implementation again.
WIP: revert commit bb52a29