An UndirectedHyperGraph may now be converted into another uHG with the
same Vertex and Edge types but another set of (Vertex-, Edge- and Graph-)
Properties

A DirectedHyperGraph may now be converted into another DHG with the same
Vertex and Edge types but another set of (Vertex-, Edge- and Graph-)
Properties

All instanciations of template class UndirectedHypergraph are friends to
one another

All instanciations of template class DirectedHypergraph are friends to
one another

All instanciations of template class Hypergraph are friends to one another

An implicitly-defined dtor is non virtual, which may lead to memory leaks

Each include dir is responsible for adding its header files to
hglib_HEADER_FILES

subrepo:
  subdir:   "lib/filtered_vector"
  merged:   "95c52f7c"
upstream:
  origin:   "git@gitlab.inria.fr:kirikomics/filtered_vector.git"
  branch:   "master"
  commit:   "120ed556"
git-subrepo:
  version:  "0.4.0"
  origin:   "???"
  commit:   "???"

Extracting the definition of these nested classes from their enclosing
class makes the whole more readable

There was no reason to return a copy, make it a const ref

Make HyperedgeBase::SpecificAttributes::Clone pure virtual to prevent
developers of derived classes from forgetting to override it

Used to be done from Derived classes init ctors

Handle DirectedHyperedge SpecificAttributes through a class hierarchy
with base class DirectedHyperedgeBase::SpecificAttributes

Handle UndirectedHyperedge SpecificAttributes through a class hierarchy
with base class UndirectedHyperedgeBase::SpecificAttributes