drop "import *" at function scopes

SyntaxWarning: import * only allowed at module level
compute: determine system parameters from H5MD file
tcf: use stable 'mergesort' to avoid mixing of the correlation blocks
purge deprecated package 'mdplot'

mdplot handles pre-H5MD files, going back to the very roots of H5MD.
msv: support H5MD v1.0, drop backwards compatibility

Refer to the thermodynamics module version 1.0.

According to this, rename "total_energy" to "internal_energy".

Drop hypervirial.
pdf, ssf: drop backwards compatibility layer
store common definitions in private module _common

The first use is for abbreviations of dataset names.
compute: process H5MD 1.0 files, drop backwards compatibility

The interaction cutoff is ignored yet and may be dropped at all. It
is used to compute tail corrections.