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.