thermodynamics writer: fix default path for global groups

"nil or something" will always yield "something" since "nil" evaluates to
false. Negating the expression puts "nil" at the end.

The bug was introduced in commit:6153630.
thermodynamics: return particle number as integer

The data slot wrapper unintentionally converted the integer to a double
which caused the H5MD writer to choose 'double' as data type.
make the thermodynamics unit test an integration test

The test suite unit/observables/thermodynamics did not only test the
functionality of the thermodynamics module, but was actually an
integration test validating the thermodynamic properties of a simple
Lennard-Jones fluid.

The test is refactored to call the simulation scripts
examples/liquid/lennard_jones.*.lua in a sequence. Afterwards, a small
C++ test case analyses the resulting H5MD file, performing the same
checks as before.

The parameters of the test correspond to the validation section in the
documentation. The tolerances are quite large, however, to avoid
excessively long run times of the tests.

Rescaling of the velocity after the equilibration run is missing. It may
not be crucial here, but could be included in the example scripts.
liquid scripts: set up wavevectors only when needed

The search for suitable wavevectors may take a significant time,
particularly for large simulation boxes.