particle groups: add property "global"

This property tells if the particle group comprises the whole simulation
world. It is used later on by the thermodynamics writer to determine the
default path in the H5MD file.
thermodynamics: comply with corresponding H5MD module v1.0

Rename total_energy to internal_energy.

Store "density" instead of "volume".

Store global observables in "/observables" rather than in a subgroup
(e.g., "all"). The distinction is based on the result of
``group.global``.

This commit refs #198804.