[Sofa.Core] Add BaseLink forwarding accessor.

Currently accessing:
```python
forcefield.mstate.position
```
does not work because forcefield.mstate is a link and thus a link does not have a "position" field.
The working version consist to write ```object.mstate.getLinkedBase().position``` which is very cumbersome to write.

This PR adds support for attribute forwarding through link allowing the firt approach.

Author: damienmarchal

bindings/Sofa/src/SofaPython3/Sofa/Core/Binding_BaseLink.cpp

@@ -26,6 +26,7 @@ using  sofa::core::objectmodel::BaseObject;
 
 #include <sofa/core/objectmodel/BaseNode.h>
 
+#include <SofaPython3/Sofa/Core/Binding_Base.h>
 #include <SofaPython3/Sofa/Core/Binding_BaseLink.h>
 #include <SofaPython3/Sofa/Core/Binding_BaseLink_doc.h>
 #include <SofaPython3/PythonFactory.h>
@@ -67,6 +68,29 @@ std::string getPathName(BaseLink& self)
     return (n ? n->getPathName() : o->getPathName()) + "." + self.getName();
 }
 
+namespace {
+py::object __getattr__(py::object self, const std::string& s)
+{
+    py::object base = getLinkedBase(py::cast<BaseLink&>(self), 0);
+    if(!base.is_none())
+    {
+        return BindingBase::__getattr__(base, s);
+    }
+    throw std::runtime_error("Unable to find attribute on an empty link. X");
+}
+
+void __setattr__(py::object self, const std::string& s, py::object value)
+{
+    py::object base = getLinkedBase(py::cast<BaseLink&>(self), 0);
+    if(!base.is_none())
+    {
+        BindingBase::__setattr__(base, s, value);
+        return;
+    }
+    throw std::runtime_error("Unable to find and set an attribute on an empty link.");
+}
+}
+
 void moduleAddBaseLink(py::module& m)
 {
     py::class_<BaseLink> link(m, "Link", sofapython3::doc::baseLink::baseLinkClass);
@@ -89,11 +113,12 @@ void moduleAddBaseLink(py::module& m)
     link.def("getLinkedBase", getLinkedBase, "index"_a = 0, sofapython3::doc::baseLink::getLinkedBase);
     link.def("setLinkedBase", &BaseLink::setLinkedBase, sofapython3::doc::baseLink::getLinkedBase);
 
-
     link.def("getLinkedPath", &BaseLink::getLinkedPath, "index"_a = 0, sofapython3::doc::baseLink::getLinkedPath);
     link.def("getPathName", getPathName, sofapython3::doc::baseLink::getLinkedPath);
     link.def("read", &BaseLink::read, sofapython3::doc::baseLink::read);
 
+    link.def("__getattr__", &__getattr__);
+    link.def("__setattr__", &__setattr__);
 }
 
 } /// namespace sofapython3

bindings/Sofa/tests/Core/BaseLink.py

@@ -95,6 +95,14 @@ def test_getLinked_and_Owner(self):
         self.assertEqual(link_input.getOwnerBase().getName(), "BarycentricMapping")
         self.assertEqual(link_output.getOwnerBase().getName(), "BarycentricMapping")
 
+    def test_baselink_attributes_forwarding(self):
+        root = create_scene("root")
+        c1 = root.addObject("MechanicalObject", name="t1", position=[[-1,-2,-3],[-4,-5,-6]])
+        f1 = root.addObject("RestShapeSpringsForceField", name="forcefield", mstate="@t1")
+        self.assertEqual(len(f1.mstate.position), 2)
+        f1.mstate.position = [[1.0,2.0,3.0]]
+        self.assertEqual(len(f1.mstate.position), 1)
+
     @unittest.skip # Segmentation fault on MacOS
     def test_read(self):
         root = create_scene("root")