message: Add warning in the doc of assign_filtration
  Apparently some users expect that it will magically "fix" other
  simplices so it remains a valid filtration.
diff: @@ -74,13 +74,22 @@ cdef class SimplexTree:
  return self.get_ptr().simplex_filtration(simplex)
  def assign_filtration(self, simplex, filtration):
  - """This function assigns the simplicial complex filtration value for a
  + """This function assigns a new filtration value to a
  given N-simplex.
  :param simplex: The N-simplex, represented by a list of vertex.
  :type simplex: list of int.
  - :param filtration: The simplicial complex filtration value.
  + :param filtration: The new filtration value.
  :type filtration: float
  +
  + .. note::
  + Beware that after this operation, the structure may not be a valid
  + filtration anymore, a simplex could have a lower filtration value
  + than one of its faces. Callers are responsible for fixing this
  + (with more :meth:`assign_filtration` or
  + :meth:`make_filtration_non_decreasing` for instance) before calling
  + any function that relies on the filtration property, like
  + :meth:`initialize_filtration`.
  """
  self.get_ptr().assign_simplex_filtration(simplex, filtration)
  