NF: __reduce__ and __repr__ in VolumeBasedSurface
BF: converting from iso-8859-15 into utf-8 for having Pólya

I believe that this is what was causing coversall to fail
Thanks Илья Барышев for the support
ENH: respect externals check for pprocess + tune up exception msg

Otherwise it becomes difficult to disable pprocess for the purpose of debugging
without uninstalling it or tricking it via PYTHONPATH etc
ENH: introduce internal _strid to unify appearance of ids (Close #116)