observables/…/accumulator: fix and clarify documentation

Revise and clarify documentation of 'writer' method, move argument
'location' up to follow 'file' directly.
observables/phase_space: specify which fields shall be read or written

This allows the user to select which data are stored in and fetched from
the /trajectory group of the H5MD output file.

Thereby, storing masses and species in the liquid examples can be
dropped in the future. A missing prerequisite is the revision of
phase_space:set() along with the splitting of the phase_space sample
into individual arrays. Currently, the mass and species values are
overwritten by phase_space:set(sample) and there is no method to restore
them to a constant value.
observables/thermodynamics: revise interface and documentation

Change names of Lua bindings to reflect the group names of the H5MD
output files. This follows the coding policy to avoid abbreviations in
interface definitions and it provides a 1:1 mapping between field names
and module methods.

Change interface of writer() analogously to phase_space:writer() such
that the user may select which observables are evaluated and written.
Provide defaults for the most commonly used fields that comply with the
H5MD specification. (Note that the total energy may be obtained from
potential_energy and temperature.)

Extend the documentation and provide formulas for each observable.