mdsim.potentials: move to sub-namespace "pair"

This prepares for the implementation of other potential types, e.g.,
external potentials.

The change breaks basically all simulation scripts, which can be fixed
by replacing "mdsim.potentials" by "mdsim.potentials.pair", etc.
doc: fix Unicode character for apostrophe

cd doc/pdf && pdflatex halmd.tex:

! Package inputenc Error: Unicode char \u8:ʹ not set up for use with LaTeX.

See the inputenc package documentation for explanation.
Type  H <return>  for immediate help.
 ...

l.8973 \emph{HALʹs MD package}
                                is maintained in a public Git repository. \h...
move from boost::array to std::array

As boost::list_of is not compatible with std::array, convert
construction from list_of to C++11 initializer lists.
C++11: move from boost::tuple to std::tuple