lua/phase_space: improve support 'memory' parameter

The memory option for potential added in 27574e5 introduced a problem
with the phase_space module, as the phase_space sample can be in gpu or
in host memory, too. But if the particle instance is in host memory, the
phase_space sample can be in the host only. Therefore, restrict the
possible phase_space sample instances depending on the memory variant of
the host.

Also add an optional memory parameter to phase_space:reader() and fix a
minor problem with the docuemnted type of some parameters of the reader.
hilbert: revise lua module

Implement the simplified connection beween lua and c++ using
std::make_shared at the luaponte definition.

Document the lua module.
lua/neighbour: fix disconnect()

The profiler connections were not added to the connection table,
therfore disconnect() would not do anything.
expand description of Hilbert sort module