Concrete types

Created by: gcarleo

This PR re-establishes the usage of concrete types for Machine, Graph etc, removing the interim usage of std::shared_ptr that was dictated by the pybind11 interface rather than NetKet design.

Concrete types are based on std::variant (actually a non-standard version backported to c++14).

This effectively moves NetKet from C++11 to C++14, which seems a reasonable move, given the current development of the compilers. Moving to C++17 is too premature because of sparse support for std::variant std::optional etc, especially on Mac platforms.

Using std::variant should (maybe) also help a little bit with performance (to be seen). Working with concrete types also has many advantages when implementing python pickling.

I am not entirely satisfied by this implementation because I wasn't able to define copy constructors for the concrete types, because of some obscure variant-related issue. Any help with this is much appreciated.

The variant-based polymorphism is obtained like this:

class Hilbert : public AbstractHilbert {
 public:
  using VariantType = mpark::variant<Spin, Boson, Qubit, CustomHilbert>;

 private:
  VariantType obj_;

 public:
  Hilbert(VariantType obj) : obj_(obj) {}

  bool IsDiscrete() const override {
    return mpark::visit([](auto &&obj) { return obj.IsDiscrete(); }, obj_);
  }
.....

Since we are working with objects, it would be useul also to use more thoroughly std::move than what I already did.