Rework Problem::{check,print} into a recursive approach

Following dicussion of issue #289 (closed) @jsola raised the concern of reworking the way Problem::{check,print} work. I reproduce his comment:

In fact, and since we are here, let me say that the functions print() and check() are horrible.

What we would need is a couple of viral functions print() and check() that:

• print the current node
• call print() on all child nodes

Moreover, each node's print would be virtual, so that derived classes can print more info / modified info , with respect to the base class.

A typical print() function in a node Parent, with children of type Child, would be split into a header section and a viral section; The Header section can be derived and so it is virtual:

virtual ParentBase::printHeader()
{
  // print relevant info of ParentBase
  // ...
}

ParentBase::print()
{
  printHeader();
  for (const ChildBasePtr& child : child_list_)
  {
     child->print();
  }
}

virtual ParentDerived::printHeader(options)
{
  FooBase::printHeader(); // print base header

  // print relevant info of ParentDerived
  // ...
}

At this point, if all Nodes in wolf have these, we can call

problem->print(options);

and obtain the same behavior as now.