Rewrite of Harmonic potential

This is an almost complete rewrite of harmonic potential. This is no longer a template but a normal base class with two inheriting classes for the simple harmonic potential and the harmonic potential with fixed centre of mass. The periodic version of the potential has been removed because it is of no use (it represents a fairly unphysical situation).
This PR also includes a bug fix to pele::array and a change to the definition of inifinity for hs_wca potential. Before merging I should at least write some c++ tests for this potential, since it currently has no tests in pele (a couple in mc pele but at a higher level).